Extracts of human benign prostatic hyperplasia, well-differentiated prostatic adenocarcinoma, and normal post-pubertal prostate stimulate 3H-thymidine incorporation by resting phase cultures of fetal rat osteoblasts and fibroblasts. The stimulation is concentration dependent and reaches a maximum at twenty-four hours of incubation. Prostatic extracts are also mitogenic in cell cultures of newborn human foreskin fibroblasts and the human cell lines, BUD-8 and DoT. The growth-stimulating factor is both heat and trypsin sensitive indicating that the factor is either a protein or contains a protein moiety. The growth-stimulating activity is not related to prostatic polyamine concentration. Experiments also show the activity is not due to human prostatic acid phosphatase. A prostatic growth factor may explain the growth of fibrous nodules in benign prostatic hyperplasia and the osteoblastic response of bone to prostatic cancer.