個乳の冷蔵中に一時的に温度が上昇した場合の遊離脂肪酸の増加を対照と比較し，それに及ぼす泌乳期の影響を調査した。試料2.3mℓを20mℓ容共栓付試験管に採り，氷水中に24時間保存した場合の遊離脂肪酸の増加（パルミチン酸としてmg1mℓ）を自然発生リポリシスとした。冷蔵開始1時間後に20℃水浴中に3分間静置し，直ちに氷水中に戻した場合の遊離脂肪酸の増加を温度活性化リポリシスとした。さらに殺菌均質化乳を基質源として，0℃およひ37℃におけるリパ一ゼ活性を測定した。
　分娩4日後までの初乳（10頭）は，両リポリシス，リパーゼ活性共に低く，温度活性化がみられないが，6日後にはいずれも常乳期初期と同程度になり，温度活性化も明確になった。
　引き続き9頭を用いて分娩後8ヵ月までの調査をおこなった。リパーゼ活性については泌乳期の影響は明確ではなく，0℃，37℃活性は，それぞれ，1.3～1.8, 23.9～45.0×10^－3mg/mℓ/minであった。リポリシスについては，リパーゼ活性よりも個体差が大きいが，泌乳期の進行にともない明らかに増大した。分娩後8ヵ月には自然発生リポリシスは1ヵ月の場合の8.6倍（146.3土142.1 mg/mℓ），温度活性化リポリシスは同じく9.1倍（318.0土293.7mg/mℓ）に達した。
　常乳期においては，温度活性化リポリシスと自然発生リポリシスの間の相関係数は高く，全牛については0.85で有意（p＜0.001）であった。温度活性化リポリシスと0℃活性，37℃活性の間の相関係数は0.61, 0.37で有意（p＜0.01, p＜0.05）であった。しかし，各個体についてみると，高い相関を示すものと全く相関を示さないものがみられた。

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　This study was designed to determine the basic characteristics of nisin-producing lactic acid bacteria. The optimal conditions of culture of this bacteria and the antibacterial effect of nisin on LD type starter bacteria, and on butyric acid bacteria which induces gas blowing during cheese ripening, was determined.
　The optimal temperature for growth of nisin-producing lactic acid bacteria was between 20 and 25℃, while that for acid production was between 30 and 35℃. High temperature was suitable for nisin production, and about 300 IU/ml of nisin was produced during cultivation of nisin-producing lactic acid bacteria in skim milk at 30℃ for 24 hrs.
　Mixed culture of LD typel actic starter and nisin-producing lactic acid bacteria inhibited the acid production of LD type starter. This inhibition was frequently observed in the early phase of cultivation, and was caused by nisin transferred from precultures of the nisin-producing lactic acid bacteria at inoculation. Thus, various extents of inhibition were observed for different sorts of precultures of the nisin-producing lactic acid bacteria and for different sizes of inoculum.
　In the LD type starter, Lactococcus lactis subsp. cremoris SC-1 and SC-3 exhibited the highest sensitivity to nisin; its growth was inhibited with about 2 IU/ml of nisin. However, this strain became resistant to extremely high concentrations of nisin when nisin content gradually increased during cultivation. The growths of L. lactis subsp. lactis biovar. diacetylactis SD-1 and SD-3, and Leuconostoc mesenteroides subsp. cremoris LC-2 were inhibited at about 5 and 20 IU/ml of nisin, respectively, while L. lactis subsp. lactis SL-2 could grow at between 50 and 100 IU/ml of nisin, and thus exhibited significant resistance to nisin. On the other hand, of the clostridia, which displayed variation in sensitivity to nisin depending on the number of inoculated spores, Clostridium tyrobutyricum KS-222 exhibited the highest sensitivity, with growth inhibition at about 20 IU/ml of nisin in the presence of 1×10² spores/ml. The growth of Cl. butyricum K-53 was inhibited at about 50 IU/ml of nisin, while Cl. sporogenes K-21 could grow even at 100 IU/ml.
　The concentration of nisin decreased during cultivation of L. lactis subsp. lactis SL-2, Cl. butyricum K-53 and Cl. sporogenes K-21, which displayed resistance to nisin, suggesting that these strains were able to inactivate nisin.

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