Chloramphenicol resistance in salmonella obtained from clinical sources in Ontario was previously found to be often mediated by R plasmids of the H2 incompatibility group. In the present study 40 salmonella strains resistant to one or more of kanamycin, streptomycin, sulfonamides, or tetracycline, but sensitive to chloramphenicol, were investigated to determine if they contained R plasmids. Self-transmissible plasmids were isolated from 17 of the strains, and 7 of these showed the bacteriophage inhibition and thermosensitive mechanism of transfer characteristic of H2 plasmids. Entry exclusion and incompatibility experiments confirmed their classification. The results demonstrate that in this population of salmonella, R plasmids of the H2 group are prevalent. Experiments with plasmid-specific phages indicate that the plasmids of this sample, which are not in the H2 group, do not belong to any of the F, I, N, P, or W incompatibility groups.