Modification of 12 arginine residues per molecule of formate dehydrogenase (formate : NAD+ oxidoreductase, EC 1.2.1.2.) from the methylotrophic bacterium, Achromobacter parvulus I, by 2,3-butanedione results in complete inactivation of the enzyme. Inactivation of the enzyme is reversible and proceeds in two steps via formation of the intermediate enzyme-butanedione complex. Coenzymes but not formate effectively protect formate dehydrogenase from inactivation. Complete maintenance of enzyme activity and specific protection of one arginine residue per enzyme subunit are achieved on formation of the binary complex, enzyme-NAD, or the ternary complex, enzyme-NAD-azide. One arginine residue is supposed to be located at the NAD-binding site of the formate dehydrogenase active centre.