Primary cultures of human breast cells prepared from surgical specimens of reduction mammoplasty were used to study the activity of the enzyme 17 beta-hydroxysteroid dehydrogenase (E2DH) which converts estradiol (E2) into its less active metabolite estrone. This study was performed in both epithelial and stromal cells separated, after collagenase digestion of the tissue, on a Percoll gradient, and then cultured as monolayers in Ham's F 10 medium supplemented differently for epithelial cells and fibroblasts. E2DH activity was strikingly higher in epithelial cells than in fibroblasts, since after [3H]E2 incubation (2 nM), 600 fmol/micrograms DNA were metabolized to estrone in epithelial cells after 1 h, whereas an equivalent amount was hardly obtained in fibroblast cultures after 24 h. The affinity and capacity of E2DH were greater in epithelial cells with apparent Michaelis-Menten constant (Km) = 0.6 +/- 0.1 microM and maximum velocity (Vmax) = 250 to 360 pmol/micrograms DNA/h, whereas they were 10 +/- 1 microM and 50 to 70 pmol/micrograms DNA/h, respectively, in fibroblast cultures. Moreover, the E2DH activity was 2 to 5 times higher in epithelial cells cultured in the presence of the progestin medroxyprogesterone acetate, whereas it remained unchanged in fibroblasts cultured under the same conditions. This increase in E2DH activity was dose dependent from 10(-10) to 10(-7) M medroxyprogesterone acetate and inhibited by both actinomycin D and cycloheximide. This system of differential breast cell culture appears to be a fruitful tool for the study of the hormone dependence of normal breast growth and differentiation. Due to the presence of E2DH, epithelial cells are more apt to undergo and to moderate E2 action. Moreover, epithelial cells are a possible site of progesterone modulation of E2DH activity. Therefore, E2DH could be a good marker both for epithelial cells and their hormone dependence.