Silymarin, a naturally acknowledged hepatoprotector used in humans to treat liver diseases has been tested in murine (HC11) and bovine (BME-UV) mammary epithelial cell lines to evaluate a possible direct effect on cell growth and differentiation in mammary gland. Silymarin enhanced cell proliferation (p < 0.05) from 10 to 1000 ng/ml in association with growth factors, (up to 20%) or alone (up to 15%) versus controls. Furthermore, silymarin (100 ng/ml) was able to increase (p < 0.05) beta-casein gene expression alone or in association with prolactin (5 microg/ml). These effects may be related with protein kinase B (AKT) activation induced by silymarin treatment (p < 0.05) and/or by a dose-related inhibitory effect (p < 0.05) on caspase-3 activity related to a protective role in cell apoptosis. These data suggest that silymarin should be considered a candidate to support mammary gland activity during a lactogenetic state.