Summary: Mutants of Escherichia coli K12 have been isolated which reduce nitrite 3 to 30% as rapidly as the wild-type. Activities of reduced nicotinamide adenine dinucleotide (NADH)-nitrite oxidoreductase were lower in cell-free extracts of these nirA * mutants than in the wild-type. The mutants grew on minimal agar, and their sulphite reductase activity was the same as in the wild-type. Double mutants deficient in both nitrite and sulphite reductases were constructed, as well as recombinants which had regained one or both activities following recombination with Escherichia coli Hfr Hayes. The inability to reduce sulphite was due to an altered cysB† gene. Suspensions of nirA cysB + and nirA cysB bacteria reduced nitrite at similar rates, showing that sulphite reductase (which is a gratuitous nitrite reductase) contributes little to the rate of nitrite reduction in vivo. Cytochome c 552 was synthesized by nirA + cysB + double recombinants but not by nirA cysB or nirA cysB + bacteria. This data suggests that cytochrome c 552 is involved in nitrite reduction in E. coli either as a component of NADH-NO2 - oxidoreductase, or as an electron carrier whose synthesis is affected by the nir gene.