1. Four to six moles of sulfhydryl groups of yeast alcohol dehydrogenase were found to be high sensitivity for 5, 5'-dithiobis (2-nitrobenzoic acid) (DTNB). 2. There were two types of sulfhydryl groups in the active site of yeast alcohol dehydrogenase. Sulfhydryl groups of two of four active sites were easily modified by DTNB at 0°and the other were not. 3. Yeast alcohol dehydrogenase which was modified sulfhydryl groups of two of four active sites by DTNB, was fully restored in its enzyme activity by cysteine and was found to be the same conformation as the native enzyme. On the other hand, the enzyme which was modified more than two sulfhydryl groups in the active sites, was partially restored by cysteine, probably due to irreversibly conformational change. 4. DTNB competed with NAD for sulfhydryl group in the active site of yeast alcohol dehydrogenase.