1. An enzyme from Escherichia coli 9723 that reduces adenosine 3'-phosphate 5'-sulphatophosphate to inorganic sulphite is described. Extracts of E. coli K(12) and Bacillus subtilis 1379 contain a similar enzyme. 2. This reductase and sulphite reductase (EC 188.8.131.52) of E. coli 9723, E. coli K(12) and of B. subtilis are repressed by growth in the presence of l-cystine. Cysteine synthase (EC 184.108.40.206) is unaffected. 3. Growth of E. coli 9723 on inorganic sulphite represses the sulphate-activating enzymes (EC 220.127.116.11 and 18.104.22.168) almost completely but has little effect on sulphite reductase. Growth on 0.042-0.056mm-l-cystine gives a similar result. 4. Such differential repression by cyst(e)ine prevents E. coli, when growing on sulphite, from synthesizing unnecessary enzymes.