An H2S-producing variant of Escherichia coli (strain 142) isolated from a urinary tract infection was found to be resistant to high levels of tetracycline, ampicillin, streptomycin, and sulfonamide. The H2S trait segregated spontaneously at a frequency of 2.5 x 10-3. No segregation was observed for the drug resistance determinants. Neither ethidium bromide nor acridine orange affected the rate of segregation of the drug resistance determinants or the trait for H2S production. Antibiotic resistance and hydrogen sulfide production were conjugally transferred to E. coli K-12 recipients at a frequency of approximately 1i-5 per donor cell. Antibiotic resistance and hydrogen sulfide production were also transduced as a single unit with phage P1L4. Genetic data, based on the segregation of resistance determinants and the H2S trait among transconjugant and transductant classes, suggested the presence of two R plasmids. Plasmid DNA was isolated by cesium chloride-ethidium bromide centrifugation. Two plasmid species were detected by agarose gel electrophoresis of purified plasmid DNA, a large molecule of about 80 x 106 daltons (designated pSR12) and a small molecular species of approximately 5.5 x 106 daltons (designated pSR13). Transformation studies using purified plasmid DNA showed that the large pSR12 plasmid confers resistance to ampicillin, tetracycline, and streptomycin and also carries the gene(s) for H2S production. The small pSR13 plasmid confers resistance to streptomycin and sulfonamide.