Gemara

(Beraisa - R. Yehudah): If someone died on a bed and a Revi'is of blood dripped (before and after death) into a cavity, it is Tamei, for the drop (that came out at the time) of death is mixed in.

Chachamim are Metaher, for every drop (after death) is Batel when it is mixed with a majority that came out before death.

R. Yehudah holds that blood is never Batel in blood.

Bechoros 22a (Reish Lakish): If one buys brine from an Am ha'Aretz, he may do Hashakah (connect it to a Mikveh). Even if the majority is brine, brine is not Mekabel Tum'ah, and the water is Batel in the majority.

(R. Yirmeyah): This does not help if he will (add water, e.g.) cook it in a pot, and altogether there will be more water than brine. Then, the Tum'ah is revived.

Avodah Zarah 73a (Mishnah): Yayin Nesech (that was mixed with other wine) forbids b'Mashehu (any amount).

(Rav Dimi citing R. Yochanan): If one pours Yayin Nesech from a barrel into a pit of wine, even the entire day, each bit poured in becomes Batel.

75a: The same way we are Metaher, we Kasher from Yayin Nesech.

Rishonim

Rambam (Hilchos Chametz 4:12): If Chametz became mixed with something that is not a food, or which most people do not eat, such as Tri'ak (a medicine containing crumbs of dry Chametz. Only kings or one bitten by a snake or rabid dog eats it), one may keep it, but he may not eat it until after Pesach. This is even if contains only a drop of Chametz.

Rosh (Pesachim 2:5): After six hours on Erev Pesach, even though one is lashed mid'Oraisa for Chametz, it is like other Isurim and it is Batel in 60 and it is permitted even during Pesach, since it became Batel before Pesach.

Tosfos (Bechoros 22a DH Yasiv): R. Yochanan holds that even Tum'as Chulin, which is lenient, is revived, but he holds that even Isur is not revived! R. Tam says that when the Isur is added constantly, it is revived. Regarding Dam Tevusah, we distinguish between whether or not it flowed constantly. However, here there was an interruption! It seems that even then, the Isur is revived. Chachamim were lenient about Dam Tevusah, which is only mid'Rabanan, and in other cases due to Dichuy. R. Yochanan was lenient about Yayin Nesech only as long as there are 60 parts of wine to be Mevatel it. He taught that the Mishnah teaches that Yayin Nesech forbids b'Mashehu, but not when it is poured into Heter. When we say 'the first is Batel', it is only as long as the majority is Heter. Perhaps Yayin Nesech is different.

Poskim

Shulchan Aruch (OC 447:4): If Chametz became mixed before Pesach and it was Batel in 60, it is not revived during Pesach to forbid b'Mashehu. Some disagree.

Beis Yosef (DH uv'Pesach): The Rosh, Mordechai, Hagahos Maimoniyos citing Semak, and Semag all permit Chametz that became Batel before Pesach. Hagahos Maimoniyos said that some forbid, but it seems that he holds like those who permit.

Taz (5): If before Pesach a wet mixture does not have 60 parts to be Mevatel the Chametz, one may add to it before Pesach to make 60. This is not Bitul Isur l'Chatchilah. I brought a proof in 626:1 (below). The Rema does not say 'wet' to exclude flour, for also it is considered wet, like the Beis Yosef brought in 453:3. Also regarding a spit, we do not say the Isur is revived (451:27).

Gra (DH v'Yesh): The stringent opinion learns from Bechoros. Tum'ah is revived, and the same applies to Isur. Tosfos and Avodah Zarah 75a equate Tum'ah and Isur. The Mordechai (550, citing Avi ha'Ezri) gives a different reason. Before Chametz is Asur, it is not Batel. One cannot immerse a Kli to be Metaher it for when it will become Tamei (Nedarim 76a). Other Isurim become Batel before they are forbidden, e.g. (sheep's wool in) camel's wool before it is mixed with linen, or up to one part in 24 of another species of seeds are Batel before planting. Chametz is different, for when it is Asur, forbids any amount; there is no Bitul. The Gemara (76b) concludes that normally, an Isur can be annulled before it takes effect. A verse teaches that Nedarim are different.

Rema: The custom is like the first opinion regarding all wet mixtures. However, we forbid if dry matters mixed, or there is concern for a mixture. E.g. if bread fell into wine, even though it was removed, we are concerned lest crumbs remained in the wine and they give taste during Pesach.

Magen Avraham (11): (All agree that) if Chametz became Batel in a dry mixture, one must eat it before Pesach, for if not, Chametz is in his house. All permit keeping a wet mixture. The Rosh (brought in Tur YD 109) says that Isur that was Batel in a dry mixture becomes total Heter. We follow the Rosh only to avoid a loss. However, if one can cook it until it dissolves and mixes before Pesach in 60, one may eat it during Pesach, and this is not being Mevatel an Isur l'Chatchilah.

Shulchan Aruch (453:3): If one did not remove from wheat the grains that mice bit, there is no problem.

Taz (2): The Tur says that even if there are bitten pieces, they are not one part in 1000, and they are Batel when it is ground. This is not being Mevatel an Isur l'Chatchilah, for he does not grind in order to be Mevatel. This is unlike Hagahos Semak, who says 'one must select well, so there be 60 times as much that did not sprout. Even though it is a dry mixture, it gives taste during the baking. Also, he must bake it before Pesach. If he bakes during Pesach, even any amount forbids. Since people normally bake during Pesach, one must be careful that there is no wheat that sprouted in the Pesach grain.' He holds that flour does not mix with flour to be like a wet mixture. Only after it is baked it becomes like one piece. Therefore, he must bake before Pesach. Also Terumas ha'Deshen (114) says that we do not hold like Semak, rather, like Semag, who says that flour mixes with flour to be like a wet mixture. Here when there is 60, one may bake it during Pesach. Since it was Batel before Pesach, it is not Chozer v'Ni'ur (revived) during Pesach, like a wet mixture (447:4). However, in Siman 466:4 the Shulchan Aruch says that if the wetness dried before Pesach, sifting does not help, for it crumbles and mixes and one may not eat it. This is from the Mordechai. The Mordechai already brought that what is Batel before Pesach is not Chozer v'Ni'ur. We must say that when it is dry, it is not Batel, for flour does not mix with flour, like Semak, unlike the Shulchan Aruch here! We must say that flour mixes with flour to be like one piece only if they were ground together. This is like baking, according to Semak. In Siman 466, it fermented after it was ground. Therefore, one may not bake it even before Pesach, for surely one will come to bake it also during Pesach.

Bi'ur Halachah (DH Im): The Mechaber connotes that l'Chatchilah one must remove the bit pieces. The Tur, Levush and Ma'amar Mordechai say that one need not. Perhaps the Mechaber discusses when there is not 60 times as much Kosher wheat, therefore l'Chatchilah one must remove it.

Kaf ha'Chayim (46): Some say that the Heter is due to Bitul, like the opinion that it is not Chozer v'Ni'ur, and we permit only until 60. The Acharonim hold that the Heter is because it is very unlikely that a small amount of mouse saliva can ferment hard wheat, therefore it is even if it is more than one part in 60.

Rema: The same applies if one did not select off the grain that sprouted. However, one must ensure that there is not so much that there is not 60 times as much Heter.

Magen Avraham (6): One need not bake it before Pesach, for flour with flour is considered a wet mixture, so it is not Chozer v'Ni'ur.

Mishnah Berurah (17): Even though grain that sprouted due to moisture of the ground is full Chametz, if he did not remove it we do not forbid, for it is Batel in the Kosher wheat. One may grind it all together. Many Acharonim permit baking it even during Pesach. Even though during Pesach any amount forbids, it was already Batel in 60 before Pesach, and flour with flour is considered a wet mixture, so it is not Chozer v'Ni'ur. However, a Ba'al Nefesh (pious person) should be stringent for himself to bake it before Pesach when some sprouted.

Mishnah Berurah (19): If there is not 60, he must remove sprouted wheat. He need not remove all of it. It suffices if he does not leave one part in 60.

Mishnah Berurah (20): If there were not 60 times as much as the sprouted wheat, some permit adding Kosher wheat in order to be Mevatel it and grind it before Pesach. This is not called Mevatel an Isur l'Chatchilah, for it is not yet Isur. Most Poskim forbid. Since he mixes with intent to eat it during Pesach, this is like Bitul when it is forbidden. In pressed circumstances one may rely on the lenient opinion.

Kaf ha'Chayim (52): The Pri Chodosh forbids grinding it together, for the Rashba (485) says that any recognizable intact Isur is not Batel, even in 1000; one can check to see which wheat sprouted. Nehar Shalom says that the Rema agrees, but permits because he does not grind in order to be Mevatel. L'Chatchilah one should be concerned for the stringent opinion and select out the sprouted wheat. B'Di'eved, one may rely on the lenient opinion. This is like the opinion that wet mixtures are not Chozer v'Ni'ur. The opinion that they are Chozer v'Ni'ur forbids even b'Di'eved.

Shulchan Aruch (626:1): If a Sukah has mostly shade only together with braches (Pasul Schach), he must lower the branches and mix them with the Kosher Schach so they are not recognized, and the Kosher Schach is the majority and it is Mevatel them.

Taz (626:2): The Shulchan Aruch permits l'Chatchilah. Normally, one may not be Mevatel an Isur l'Chatchilah. The Levush says that this is a decree lest one permit the entire Isur. This does not apply to a Mitzvah. He seeks to do a Mitzvah, so he will not do so through an Aveirah! I say that there is no question at all. It was never called Isur! Surely, before Sukos there is no Isur, for there is no Chiyuv Sukah. Even during Sukos, there is no Isur due to the Sukah, just one who sits there fulfill the Mitzvah. The Bach brings from the Mordechai and Agudah that the Isur to be Mevatel an Isur is only mid'Rabanan. It is only when one benefits, (so it does not apply to Mitzvos, for) Mitzvos Lav Leihanos Nitnu. Therefore, l'Chatchilah one may add to a wet mixture of Chametz before Pesach so there will be 60, since there is not any Isur yet.