Listeria monocytogenes isolates resistant to 10(5) IU ml-1 nisin were obtained at 30 degrees C (NR30) and at 10 degrees C (NR10). Nisin prolonged the lag phase of isolate NR30 at 10 degrees C. Isolates NR30 and NR10 did not produce a nisinase. Protoplasts of isolate NR30 were unaffected by exposure to nisin. The fatty acid composition from the wild-type strain and NR isolates was determined. As expected, temperature-induced differences in the C15/C17 fatty acid ratios were found. Growth of the NR strains in the presence of nisin resulted in significantly different C15/C17 ratios and a significant increase in the percentage of C16:0, C16: 1, C18:0 and C18: 1 fatty acids at 10 degrees C and 30 degrees C. Both the NR10 and NR30 isolates had similar growth rates at low temperatures, but these were slower than the wild-type strain. These results indicate that 'nisin resistance' is an environmentally defined phenotype and that nisin induces changes in the fatty acid composition of the membrane in L. monocytogenes nisin-resistant isolates regardless of the growth temperature.