Microorganisms able to form L-cysteine from DL-2-amino-delta2-thiazoline-4-carboxylic acid (DL-ATC), a chemical intermediate in the synthesis of DL-cysteine, were isolated from soil samples and classified as Pseudomonas sp., Pseudomonas cohaerens, P. desmolytica, and P. ovalis. Thirteen L-cysteine-producing bacteria were also found in among 463 stock cultures representing 37 genera. These were Achromobacter delmarvae. Alcaligenes denitrificans, Bacillus brevis, Brevibacterium flavum, Enterobacter aerogenes, Erwinia carotovora, Escherichia coli, Micrococcus sodonensis, Myocoplana dimorpha, Sarcina lutea, Serratia marcescens, Flavobacterium acidoficum, and Pseudomonas ovalis. In the presence of intact cells of Pseudomonas sp. AJ 3854, 6.1 mg of L-cysteine and/or L-cystine per ml was produced from 10 mg of DL-ATC-3H2O per ml in a molar yield of 100%. This finding suggests that racemization and asymmetric hydrolysis occurred simultaneously in this incubation mixture. After the complete oxidation of cysteine to cystine by aeration in the presence of ferrous ion, crystalline cystine was isolated; its configuration was the L isomer based on data from X-ray diffraction, microbioassay, and optical rotation studies.