We examined the effect of prostate cell extracts on the replication of highly enriched rat ventral prostate stromal and epithelial cells cultured in RPMI-1640. Extracts from normal rat prostates completely inhibited cell division in both fractions, while a 10% (v/v) extract of Dunning R-3327G adenocarcinoma inhibited replication of stromal cells but permitted that of epithelial cells. The cytotoxic effect of prostate extracts was dialyzable, heat stable, unaffected by proteases, soluble in acid/alcohol, and insoluble in chloroform. These properties are consistent with those of polyamines incubated in the presence of fetal calf serum. Dialyzed extracts from Dunning adenocarcinomas, and from human benign hypertrophic and carcinomatous prostates stimulated rat prostate stromal and epithelial cell division, in keeping with other reports of "growth factors" in prostate tissue. This mitogenic activity was stable to temperature (70 degrees C, 4 hours), and marginally if at all affected by exposure to trypsin or to pronase coupled to Sephadex. Short-term culture of separated prostate cells should provide a useful assay system for detecting putative autocrine or paracrine stromal and/or epithelial cell growth factors and identifying suspected homo- or heterotypic cellular interactions in normal and diseased prostates.