The pattern of antimicrobial resistance was determined for 254 strains of Salmonella isolated from clinical sources. Thirty-five strains were found to be markedly resistant to ampicillin, chloramphenicol, tetracycline, streptomycin, or sulfisoxazole. Twenty-five of the resistant strains were capable of transferring resistance to a sensitive strain of Escherichia coli during mixed cultivation. Resistance for one, two, three, or four antimicrobial compounds could be transferred. The levels of antimicrobial resistance transferred to the recipient strains of E coli were identical with those of the donor strains of Salmonella. It is apparent from these and other observations that Enterobacteriaceae capable of transferring resistance factors for single or multiple antimicrobial drugs by bacterial conjugation are widespread in the environment.