In situ hybridization and immunocytochemistry studies have shown the in vivo expression of platelet-derived growth factor B (PDGF-B) and PDGF receptor (PDGF-R) beta mRNAs and their respective protein products in the malignant epithelial cells of eight primary human prostatic adenocarcinomas. Examination of five nonmalignant adjacent prostate tissues did not demonstrate significant expression of PDGF B and PDGF-R beta mRNAs or production of their respective protein products in nonmalignant epithelial cells. Expression of androgen receptor mRNA was shown to be present in the epithelial cells of all of the five nonmalignant adjacent prostate tissues. There was a significant reduction in the expression of androgen receptor mRNA in poorly differentiated regions, and a moderate reduction in the well differentiated regions of the malignant tissues. It appears that dedifferentiation of the tumor cells in prostatic adenocarcinomas is accompanied by a reduction in androgen receptor mRNA expression. The coexpression of PDGF and its receptor in the malignant epithelial cells of prostatic adenocarcinomas signifies an abnormal autocrine loop that may contribute to their growth and maintenance.