Growth and morphogenesis of the prostate involves mesenchymal-epithelial interactions. Transforming growth factor-beta 1 (TGF-beta1) is one growth factor that may play a role in these paracrine interactions. We have localized TGF-beta1 by molecular and immunohistochemical analysis in the developing mouse prostate. Accumulations of TGF-beta1 protein were localized in the mesenchyme surrounding ductules in fetal and neonatal prostate. Previous studies in the mouse prostate reconstitution (MPR) model system have localized accumulations of TGF-beta1 to regions of oncogene-induced abnormalities. In surgically excised adult human prostate tissues, localized accumulations of TGF-beta1 are associated with prostate cancer and benign prostatic hyperplasia (BPH). Intracellular TGF-beta1 was more often associated with stromal cells in BPH and with neoplastic epithelial cells in prostate cancer. The production and accumulation of TGF-beta1 appears to involve interactions between mesenchymal and epithelial cells. Further experimental studies may clarify the relationships between TGF-beta1 and abnormal prostatic growth.