Abstract: With the aid of cellulose chromatography and preparative electrophore-sis, a purified cofactor-deficient MoFe-protein has been obtained from A. V.mutant strain uw-45 with a yield of 18.4%.After reconstitution of the cofactor-deficient MoFe-protein with the FeMoco,a new active protein was formed with a molecular weight of 220,000.The active protein could reduces CH = CHto CH₃=CH₂ having a specific activity of 5.85nM/min · mg protein as much as 22.7 times of the extracts of A.V.mutant strain uw-45. Similar to the MoFe-protein,the migration of the active protein during the course of electrophoresis was found to be slower than that of the cofactor-deficient MoFe-protein. Elemental analysis showed that the iron content was twice as much as that of the cofactor-deficient MoFe-protein. The visible absorption spectrum of the active protein was found to be identical with that of MoFe-protein. On the basis of the above results we postulate that the cofactor-deficient MoFe-protein can be transformed into MoFe-protein by combining with the FeMoco.