In the presence of dithionite, two kinds of molybdothiol complexes, particularly isolated Mo-cysteine complexes, used as models for xanthine oxidase or aldehyde oxidase exhibited catalytic activity on the reduction of nitrofurazone to its amino derivative. Of the two Mo-cysteine complexes, the activity of oxo-bridged one was apparently greater than that of sulfido-bridged one. The promoting effect was hardly shown by added cofactors or their replacements of the enzymes. The catalyzed reduction is considered to take place by consecutive direct two-electron transfer mechanism from catalytically active reduced form of the molybdothiol complexes to nitrofurazone and the probable intermediates.