Because of the influence of transforming growth factor-alpha (TGF alpha) on the cell growth in other cancer cell systems, we investigated the growth-regulatory role of TGF alpha in human prostate cancer cells. TGF alpha (5 ng/ml) stimulated LNCaP cell growth in monolayer to 60% of the level seen with dihydrotestosterone (DHT). Both DHT and TGF alpha increased cloning in soft agar twofold above that in controls. Metabolism of thymidine and uridine was also increased as evidenced by increased uptake of these macromolecule precursors. In addition, intracellular signalling as indicated by phosphatidyl inositol turnover was also increased by TGF alpha and DHT. Conditioned media contained TGF alpha by radioimmunoassay (RIA), transforming activity by rat kidney fibroblast (NRK) colony formation, and epidermal growth factor (EGF) receptor competable activity by radioreceptor assay. EGF receptors were present by binding assay and immunoprecipitation. These data demonstrate the presence of an autostimulatory growth loop in hormone-responsive human prostate cancer cells.