Crystalline cortisone reductase prepared from Strephtomyces hydrogenans has 3α‐ and 20β‐hydroxysteroid: NAD oxidoreductase activity. The total rate of reaction for mixtures of 17 α, 21‐dihydroxypregn‐4‐ene‐3,11,20‐trione with 5α‐androstan‐3,16‐dione, and of 17β‐hydroxy‐1α‐methyl‐5α‐androstan‐3‐ one with either of these compounds indicated that these substrates do not react independently. 3β‐Hydroxy‐5α‐androstan‐16‐one inhibited the reduction of both 17α, 21‐dihydroxypregn‐4‐ene‐3,11,20‐trione and 5α‐androstane‐3,16‐dione.20β‐Hydroxypregn‐4‐en‐3‐one also inhibited the reduction of both these substrates. The findings allow the view that 3α‐ and 20β‐activities may depend upon a single active centre with complex binding characteristics.