Cell cultures of fetal rat osteoblasts and organ cultures of fetal rat calvaria were used to study the effects of extracts of prostatic tissue on bone cell growth. Extracts of benign prostatic hyperplasia, but not an undifferentiated prostatic carcinoma, stimulated the incorporation of 3H-thymidine and 14C-proline into fetal rat osteoblasts, calvaria and skin fibroblasts. Extracts of well-differentiated prostatic cancer and normal postpubertal prostate also stimulated 3H-thymidine incorporation by fibroblasts. A potentially unique factor found in prostatic tissue may induce the osteoblastic response of bone to metastatic prostatic cancer.