Extracts of human prostatic carcinoma and of human benign prostatic hyperplasia were found to exhibit mitogenic activity in freshly isolated rat osteoblasts, rat fibroblasts, and osteoblast-derived osteosarcoma cells. This activity was markedly reduced by tryptic digestion of extracts, indicating its protein nature. Results of bioassay studies suggested that the prostatic material was different from a number of known growth factors. Analysis by reversed-phase high performance liquid chromatography revealed that extracts contained mitogenic activity for osteoblasts that was distinct from fibroblast-stimulating activity. Such osteoblast growth factors could form the basis of the skeletal osteogenic response to metastatic prostatic adenocarcinoma.