The sensitivity of nine strains of Listeria to nisin was determined as well as the minimum inhibitory concentration of nisin necessary to completely inhibit growth of these strains. All strains tested were variably sensitive to nisin and different MIC values were obtained, ranging from 740 to 10(5) IU/ml in trypticase soy agar and from 1.85 to 10(3) IU/ml in MRS agar. The inhibition of L. monocytogenes ATCC 7644 in TSB trypticase at different pH values and in sterilized and nonsterilized cottage cheese by nisin (37 X 10(2) IU/ml and 2.55 X 10(3) IU/g, respectively) also was investigated. This bacterium was completely inhibited after 24 h at pH 5.0 and above. At pH 4.5, 4.0, and 3.5, it was inhibited within 24 h. In cottage cheese no Listeria survivors were found at 24 h at 37 and 4 degrees C whether or not the cheese had been sterilized when as many as .35 X 10(6) cell/g were added at zero time.