Peripheral venous blood and erythrocytes from normal men and women, as well as from dogs, rabbits and sheep were incubated with 14C-labeled progesterone*, 20α-dihydroprogesterone, androstenedione and testosterone. The presence in blood and erythrocytes of a 20α-hydroxysteroid dehydrogenase activity, catalyzing the interconversion progesterone ⇄ 20α-dihydroprogesterone, and of a 17β-hydroxysteroid dehydrogenase activity, catalyzing the interconversion androstenedione ⇆ testosterone was observed. Incubation with washed erythrocytes in the presence of glucose and several co-factors favoured the formation of the reduced compounds: 20α-dihydroprogesterone or testosterone. Incubations with washed erythrocytes, without addition of glucose and co-factors, favoured the formation of the oxidized compounds: progesterone and androstenedione. Incubation of steroids with whole blood, resulted in metabolism of progesterone to 20α-dihydroprogesterone and of androstenedione to testosterone. The formation of the product during incubation in vitro increased linearly with time of incubation (15—180 min). Incubations of 20 ml blood or the equivalent amount of erythrocytes with substrate amounts of steroids varying from 0.5 to 3000 μg, gave a linear increase in the amount of product formed. The possible significance of these observations in vitro for steroid metabolism in vivo is discussed.