The sensitivity to nisin of 27 strains of Listeria monocytogenes, four of L. innocua and one of L. ivanovii was estimated at pH 6.8 and pH 5.5. Strains of L. monocytogenes showed differences in sensitivity which were not correlated with serotype. Strains of L. innocua were as resistant as the most resistant strains of L. monocytogenes, whereas the strain of L. ivanovii was relatively sensitive. Two of the most resistant strains of L. monocytogenes multiplied in aerated liquid medium adjusted to pH 5.0 with HCl, incubated at 20 degrees C; nisin, 500 IU ml-1, prevented multiplication and caused death. Following inoculation of a resistant strain into long-life cottage cheese, pH 4.6-4.7, the number of viable L. monocytogenes decreased approximately 10-fold during storage at 20 degrees C for 7 d; addition of nisin, 2000 IU g-1, to the cottage cheese increased the rate of inactivation to approximately a 1000-fold decrease in 3 d.