SUMMARY: Hydrogen sulphide production by washed bacterial suspensions in buffered substrate solutions was examined. The suspensions were tested for H,S production from cysteine, cystine, homocystine, methionine, mercaptoacetate, sulphite, sulphate or thiosulphate. Some strains of the Proteus, Klebsiella, Salmo- nella, Arizona and Bethesda groups produced hydrogen sulphide when loll or more washed organisms/ml. were incubated at 37" without an added source of sulphur. Generally tenfold greater quantities of bacteria were required to produce hydrogen sulphide within 24 hr. from homocystine, sulphite or thiosulphite as com- pared with cystine or cysteine. For the production of hydrogen sulphide from sodium mercaptoacetate, 10- and 100-fold larger quantities of bacteria were required. In the Brucella group all strains behaved alike, except two non-smooth strains which were spontaneously agglutinated in the buffer + substrate solutions and did not produce hydrogen sulphide. Among the Enterobacteriaceae the most active strains belonged to the Proteus group, the most inactive strains to the Large- Sachs group. All strains of the Arizona and Bethesda groups, and some Klebsiella, Salmonella, Serratia and Providence strains multiplied on cysteine and cystine + buffer solutions, utilizing for growth the split products, pyruvic acid and ammonia. Hydrogen sulphide production was inhibited by penicillin or streptomycin. Non- multiplying micro-organisms exposed to penicillin, streptomycin, aureomycin or chloramphenicol for 24 hr. at 37' remained viable, but lost their ability to produce hydrogen sulphide. This ability was regained on subculture. The hydrogen sulphide production of bacteria inactivated by penicillin was restored when the cells were removed by centrifugation, and residual penicillin destroyed by penicillinase. Addition of bacterial extracts heated at 50" or pyridoxal phosphate, separately or together to inactivated bacteria, reactivated the hydrogen sulphide production of Proteus vulgaris. Bacterial extracts heated at 100' exerted sIight reactivating effects. Hydrogen sulphide production by multiplying and non-multiplying bacteria has been described by many authors (see Clarke, 1953). Mention should be made of some investigations on factors which inhibit or stimulate the pro- duction of hydrogen sulphide. Beijerinck (1901) was the first to determine that many micro-organisms, among them escherichias and klebsiellas, were able to produce hydrogen sulphide from sulphite, thiosulphate or elementary sulphur, and that these reactions were more marked under anaerobic conditions. A further observation of Beijerinck (1904) was the ' indirect formation of sulphides ' by Escherichia and KZebsieZZa spp., i.e. the biosynthesis of sulphur- containing organic compounds from sulphate as S source and the production of sulphides by their decomposition. Braun & Silberstein (1942) reported that hydrogen sulphide production from cystine, thiosulphate or elementary sulphur by Enterobacteriaceae was stimulated by the presence of glucose ; the reaction