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Cysteine desulfhydrase catalyzes /^-replacement, the reverse reaction of a,/?-elimination, as well as a,^-elimination. These reactions were studied with intact cells of Aerobacter aerogenes 1-3-2 and Aerobacter cloacae IFO 12009 containing cysteine desulfhydrase. L-Cysteine and its analogues were synthesized by replacement and reverse reactions using intact cells. /?-Chloro-L-alanine, L-cysteine, S-methyl-L-cysteine, S-allyl-L-cysteine and L-serine were used as substrates together with hydrogen sulfide and methyl mercaptan to synthesize L-cysteine and Smethyl-L-cysteine via replacement reaction by intact cells. L-Cysteine synthesized from /?-chloro-Lalanine was confirmed to be entirely in L-form after isolation and identification of the product. The reverse reaction for synthesis of L-cysteine and S-methyl-L-cysteine from hydrogen sulfide or methyl mercaptan, pyruvate and ammonia was also catalyzed by intact cells. /?-Chloro-L-alanine was found to be the best substrate for synthesis of L-cysteine and S-methyl-L-cysteine by /^-replacement reaction.