The European Suspension Test was used to assess the relative resistance of 19 individual Listeria monocytogenes genotypes, isolated from the poultry processing environment, to three commercially used disinfectants employed in the plant at the time of their isolation. To establish the relative resistance between the strains, the concentration of each disinfectant was reduced until inter-strain variation became apparent. For Darasan 214 and 7058, variation was detected at 0·1% and 0·5% v/v, respectively, while Daraclean 7361 had to be reduced to only 2·5% v/v. At these concentrations, the mean microbiological effect (ME) of each disinfectant ranged between 4·3 and 3·1 log10 reduction in cfu ml-1. Significant differences between the strains were obtained with respect to their resistance to the disinfectants employed(P < 0·01), but the overall log10 reduction for genotypes 'A1' and 'A2', which were found to persist in the poultry processing environment, were not found to be significantly different from the genotypes which had been isolated on a more sporadic basis(P > 0·05). The L. monocytogenes strains fell into four groups with respect to incidence and size of plasmids isolated. The first group contained strains which carried two plasmids (5 and 40 MDa) and the other three (groups 2, 3 and 4) comprised strains which carried a single plasmid (14, 47 and 52 MDa, respectively). There was no correlation between persistent and sporadic strains with respect to incidence and size of plasmids isolated. Moreover, the strains which carried no plasmids were found to be as resistant to the disinfectants as those which did carry plasmids, suggesting that the plasmids isolated did not confer resistance of L. monocytogenes planktonic cells to the disinfectants tested. Therefore, it is unlikely that the strains which had been found to persist in the poultry processing environment did so by means of plasmid-mediated resistance to the commercial disinfectants used.