A soluble form of NADH-cytochrome b5 reductase (NADH: ferricytochrome b5 oxidoreductase, EC 126.96.36.199) was found in the cytosolic fraction of rabbit liver. The partially purified enzyme was strictly specific for NADH. It catalyzed the reduction of several substrates such as the methemoglobin-ferrocyanide complex (Hegesh, E. and Avron, M. (1967) Biochim. Biophys. Acta 146, 91-101) (apparent Km: 8 micrometer), potassium ferricyanide (apparent Km: 10 micrometer) and ferricytochrome b5 (apparent Km: 15 micrometer). Upon acrylamide gel isoelectro-focusing followed by specific staining, the enzyme was resolved into four bands (isoelectric pH: 7.05, 6.70, 6.50 and 6.30). The optimum pH of activity with ferricytochrome b5 as a substrate was 6.5. The estimated molecular weight was 25 000--30 000. The enzyme was unsensitive to cyanide. It was strongly inhibited by p-hydroxymercuribenzoate. The cytosolic liver cytochrome b5 reductase was immunologically related to the soluble cytochrome b5 reductase from human and rabbit red-cells, and to the microsomal cytochrome b5 reductase from rabbit liver.