BACKGROUND. Inhibition of protein kinase C (PKC) and modulation of transforming growth factor-โค (TGF-โค) are both associated with tamoxifen treatment, and both appear to be important in the regulation of prostate cancer cell growth. Investigations were performed which sought to measure the efficacy, and to elucidate the mechanism of growth inhibition by tamoxifen, in hormone-refractory prostate cancer. METHODS. Growth assays were performed on PC3, PC3-M, and DU145 prostate cancer cells. TGF-โค was measured by ELISA; p21 waf1/cip1 and retinoblastoma (Rb) protein levels were measured by Western blot; PKC activity was measured by kinase assay; and effects upon cell cycle were measured by flow cytometric analysis. RESULTS. IC 50 s for growth inhibition ranged from 5.5-10 M, and were not affected by estrogen. Tamoxifen-mediated growth inhibition was not associated with induction of TGF-โค. However, tamoxifen treatment was associated with inhibition of PKC, which was followed by induction of p21 waf1/cip1 , Rb dephosphorylation, and G1/S phase cell cycle arrest. Similar effects were observed with the known PKC inhibitor, Ro31-8220. CONCLUSIONS. These data suggest that micromolar concentrations of tamoxifen inhibit prostate cancer cell growth by inhibition of PKC, resulting in induction of the p21 waf1/cip1 protein.