Prostatic cancer typically produces osteoblastic metastases which are not attended by marrow fibrosis (i.e., osteoblast but not stromal fibroblast proliferation). In the present study we sought to test the hypothesis that prostatic cancer cells produce factor(s) which act selectively on human osteoblasts. Such a paracrine mechanism would explain the observed increase in osteoblasts, unaccompanied by an increase in marrow fibroblasts. To test this hypothesis we investigated the mitogenic activity released by the human prostatic tumor cell line, PC3. PC3 cells have been reported previously to produce mitogenic activity for cells that was relatively specific for rat osteoblasts compared to rat fibroblasts. However, the effects of this activity on human cells has not been examined previously. PC3-conditioned medium (CM) (5-50 micrograms CM protein/ml) stimulated human osteoblast proliferation by 200-950% yet did not stimulate human fibroblast proliferation [( 3H]thymidine incorporation). PC3 CM also increased cell numbers in human osteoblast but not fibroblast cell cultures. To determine whether the osteoblast-specific mitogenic activity could be attributed to known bone growth factors, specific assays for these growth factors were performed. PC3 CM contained 10 pg insulin-like growth factor (IGF) I, less than 2 pg IGF II, 54 pg basic fibroblast growth factor, and 16 pg transforming growth factor beta/microgram CM protein. None of these growth factors alone or in combination could account for the observed osteoblast-specific PC3 cell-derived mitogenic activity. Furthermore, when 5 micrograms/ml PC3 CM was tested in combination with maximally effective concentrations of either basic fibroblast growth factor, IGF I, IGF II, or transforming growth factor beta, it produced an additive effect suggesting that PC3 CM stimulates osteoblast proliferation by a mechanism independent of these bone mitogens. Biochemical characterization supported the hypothesis that the PC3 cell growth factor was unique from other growth factors. The PC3 growth factor did not bind to heparin and was resistant to acid as well as the reducing agent, dithiothreitol. Sephadex G-75 and fast protein liquid chromatography Mono S cation-exchange chromatography revealed the PC3-derived mitogen to be an Mr 26,000-30,000 basic protein. Therefore, we conclude that PC3 cells release a mitogen which exhibits higher specificity for human osteoblasts than human fibroblasts and is unique from other growth factors tested. Production of this mitogen by human prostatic carcinoma cells could play an etiological role in the intense osteoblast-specific stimulation that occurs at sites of bone metastases.