The properties of delta-5-3beta-hydroxysteroid dehydrogenase and 17beta-hydroxysteroid dehydrogenase in the human testis were examined using cell-free homogenates with added cofactors. Michaelis constants of the delta-5-3beta-hydroxysteroid dehydrogenase enzyme at 37 C and pH 7.4 were 8.2 times 10 minus 7M for dehydroepiandrosterone and 2.9 times 10 minus 6M for androstenediol. The optimal pH for both substrates was approximately 8.15. Dehydroepiandrosterone and androstenediol are competitive substrates for the enzyme. When free and conjugated C19 steroids in the order of 10 minus 6 were added, androstenedione and testosterone inhibited the enzyme activity for dehydroepiandrosterone while the activity for androstenediol was inhibited by addition of dehydroepiandrosterone and its sulfate as well as by androstenedione and testosterone. 17beta-Hydroxysteroid dehydrogenase had two apparent Michaelis constants for dehydroepiandrosterone, 3.3 times 10 minus 6M at low substrate concentrations and 1 times 10 minus 5M at high substrate concentrations. The enzyme activities for dehydroepiandrosterone and androstenedione were found to be enhanced by addition of the 17beta-hydroxysteroids examined and slightly inhibited by addition of dehydroepiandrosterone-sulfate and androstenediol-3-monosulfate. Androstenedione caused an inhibition of the 17beta-hydroxysteroid dehydrogenase for dehydroepiandrosterone. The interconversion between androstenedione and testosterone by the enzyme favored testosterone formation. Following simultaneous incubation of 3H-dehydroepiandrosterone and 14C-androstenediol in equal amounts, initially more testosterone was produced from dehydroepiandrosterone than from androstenediol under the conditions employed, while subsequently with accumulation of androstenediol more testosterone was produced from androstenediol.