Abstract Mycobacterium vaccae 10 growing in methanol medium synthesizes two inducible alternative NAD +-dependent formate dehydrogenases (FDH). In the presence of molybdenum, the dominating form of the enzyme is FDHI with M r 440 kDa and K m 0.32 mM for sodium formate. FDHI reduced ferricyanide as well as NAD +, and it was reversibly inactivated by formate. NAD + stabilized FDHI against this inactivation. Under conditions of artificial molybdenum deficiency (tungsten in the medium), the second enzyme (FDHII) appeared with M r about 93 kDa and K m 8.3 mM for sodium formate, and no FDHI activity was detected. FDHII did not reduce ferricyanide and was not inactivated by formate. The activity of FDHI was restored in tungsten-grown cells by pulse addition of molybdenum under conditions of blocked protein synthesis, suggesting the pre-existence of inactive apo-FDHI.