Epidermal growth factor (EGF)-like proteins may be important in regulating the growth of human prostate cancer cells and in the development of hormone independence. In the present study, we demonstrated that the DU 145 human prostate cancer cell line secretes EGF-like polypeptides into serum-free culture medium. The production of both authentic EGF and transforming growth factor-alpha was demonstrated by specific radioimmunoassays. In addition to 6-7k monomers, immunoreactive higher molecular weight species were isolated by gel chromatography. The DU 145 cells also specifically bound human EGF, with both high- (Kd 1.8 x 10(-10) M) and low- (Kd 1.1 x 10(-9) M) affinity binding sites. Exogenous EGF stimulated 3H-thymidine incorporation when cells were plated at low density in serum-free culture medium, while at higher cell density neither cell division nor 3H-thymidine incorporation was significantly altered. We postulate that DU 145 cells may be autologously stimulated by endogenously produced EGF and related polypeptides, which, under normal culture conditions, precludes any further effect of exogenous EGF.