We have studied effects of estradiol on primary cultures of nonmalignant human mammary tissue collected surgically from fibroadenomas or during reduction mammoplasties. After enzymatic digestion, "organoids" made of epithelial cells organized in ductal or alveolar structure were grown in primary cultures (up to 12 days) on different substrata (glass, plastic, collagen-coated plastic, and floating collagen membranes). Transmission and scanning electron microscopy showed that these organoids were responsive to physiological concentrations of estradiol. Condensed chromatin of epithelial cells became dispersed following estrogen treatment. The plasma membrane of epithelial cells at the surface of the organoids was dramatically modified by estradiol, which increased the number and the length of the microvilli, as observed previously in the MCF7 breast cancer cell line (Vic et al., Cancer Res., 42: 667-673, 1982). This effect was not observed with the same concentrations of progesterone, dexamethasone, dihydrotestosterone, or 1 microM tamoxifen or in fibroblasts of the same tissue, demonstrating that epithelial mammary cells are specifically responsive to estradiol. By contrast, no effect of estradiol could be evidenced on the [35S]methionine-labeled proteins released into the medium by the organoids. The estrogen-regulated protein of Mr 52,000 was not found in the medium after purification by concanavalin A-sepharose or immunoprecipitation with specific antibodies to the Mr 52,000 protein from MCF7 cells. We conclude that nonmalignant mammary cells are responsive to estrogens in primary culture.