Androgens have been found in mammary epithelium and in milk throughout the cycle of the mammary gland in vivo. The aim of this study was to investigate the possible role of these substances in mammary epithelial growth and differentiation in the mouse HC11 cell line. Cells were stimulated with testosterone, dihydrotestosterone, androstenedione and 5α-androstane-3α, 17β-diol at concentrations ranging between 0· 3 nM and 30 nM. Cyproterone acetate or flutamide, androgen receptor antagonists,(3 µM) were used to block specific androgen effects. Proliferative effects were measured by an MTT (tetrazolium blue) conversion test and [3H] thymidine uptake. HC11 cells were transfected with pβcCAT, a chimeric rat β-casein gene promoter–chloramphenicol acetyl transferase (CAT) gene construct and CAT ELISA was used to determine gene expression. RT-PCR was performed to detect androgen receptor expression. After 24, 48 and 72 h androgens significantly (P< 0· 05) increased proliferation. Androgen antagonists significantly (P< 0· 05) reduced the proliferative effects. Furthermore androgens potentiated the lactogenic effect of prolactin, insulin and dexamethasone (P< 0· 05). Finally, the androgen receptor gene was expressed in both proliferating and differentiated HC11 cells. These observations lead us to hypothesize an activity of this class of steroids in mammary physiology. In particular, androgens stimulate cell proliferation and β-casein gene expression; this influence appears to be mediated by androgen receptors.