Listeria monocytogenes NCTC 5105 and F6861 were evaluated for sensitivity to nisin. The results confirmed those previously published indicating marked differences in the sensitivity of the two strains. Mutants with increased resistance to nisin could be isolated from the less sensitive F6861 strain at a frequency of 10(-6) to 10(-7). Using a nisin specific enzyme-linked immunosorbent assay, the adsorption of nisin to the different strains and the resistant mutant were studied. At a range of nisin concentrations the amount adsorbed was found to reflect the sensitivity of the strain to nisin, with the more sensitive cells showing greater adsorption. In resistant cells, K+ efflux increased at a slower rate with increasing nisin concentration and had a lower maximum rate indicating that the mechanism of resistance involves a reduction in the accessibility or presence of suitable adsorption sites.