Two acidic and three basic forms of monomeric dihydrodiol dehydrogenase with molecular weights in the range of 36,000-39,000 were purified from human liver. One acidic enzyme (pI 5.2), which was specific for NADP- and dihydrodiols of benzene and naphthalene, was immunologically identified as aldehyde reductase. The other four enzymes oxidized alicyclic alcohols as well as the dihydrodiols using both NADP+ and NAD+ as cofactors, but showed differences in specificity for hydroxysteroids and inhibitor sensitivity. Two of the basic enzymes (pI 9.7 and 9.1) exhibited a 20 alpha-hydroxysteroid dehydrogenase activity and sensitivity to 1,10-phenanthroline, whereas the third basic enzyme (pI 7.6) oxidized some 3 alpha-hydroxysteroids at low rates and was inhibited by cyclopentane-1,1-diacetic acid. Another acidic enzyme, which accounted for the largest amount of enzyme activity in the tissue and appeared in two heterogenous forms with pI values of 5.9 and 5.4, showed a high 3 alpha-hydroxysteroid dehydrogenase activity and was the most sensitive to inhibition by medroxyprogesterone acetate. The Km values of the enzymes, except the pI 5.2 enzyme, for hydroxysteroids (10(-6) to 10(-7) M) were lower than those for xenobiotic alcohols.