The DU145 human prostate cancer cell line was shown to possess type I insulin-like growth factor receptors (IGFR). The addition of either IGF-I or IGF-II, but not insulin, to serum-free culture medium increases the rate of thymidine incorporation by the cells, a response which is suppressed by specific blockade of the previously described epidermal growth factor (EGF) autocrine growth regulatory loop. The DU145 cells secrete into conditional medium a specific IGF-binding protein (IGFBP) precipitated by an antibody to IGFBP-1, and whose secretion is also suppressed by interruption of the EGF autocrine loop. This IGFBP may modulate the bioactivity of IGFs arising from endocrine or paracrine sources in vivo. After removal of IGFBPs from the conditioned medium, no secretion of either IGF-I or IGF-II by this prostate cancer cell line is detected by radioimmunoassay and radioreceptor assay, respectively.