Androgen independent prostate cancer is recognized as a chemotherapy resistant disease. Human prostate carcinoma DU-145, LNCaP and PC-3 cells in monolayer in exponential growth were exposed to various concentrations of melphalan, 4-hydroperoxycyclophosphamide or adriamycin for 1 hour. These cells were all responsive to the drugs, with DU-145 cells being the least sensitive and PC-3 cells the most sensitive. When the three human prostate carcinoma cell lines were grown as xenografts in nude or SCID mice and the animals treated with single doses of melphalan, cyclophosphamide or adriamycin, the tumors were not very responsive to the drugs. The DU-145 tumors were highly resistant to each drug. The PC-3 tumors were more sensitive; however, even the PC-3 tumors were less drug responsive than several murine tumors. All three prostate cell lines secreted transforming growth factor-beta (TGF-beta) into the cell culture medium, and when grown as xenograft tumors increased the plasma levels of TGF-beta in the animals. DU-145 cells produced the most TGF-beta and LNCaP cells produced the least. After administration of single doses of each of the chemotherapeutic agents to animals bearing the prostate carcinoma xenografts, there was a time dependent increase in plasma TGF-beta that was greatest in animals bearing the DU-145 tumor and least in animals bearing the LNCaP tumor. Immunohistochemical staining, showed that PC-3 tumors tended to have the most intense staining for TGF-beta and LNCaP tumors the least. In situ hybridization for TGF-beta mRNA showed an increase in TGF-beta mRNA that was time independent after chemotherapy administration in all three tumors. These results support the hypothesis that the drug resistance of prostate carcinoma is manifest in vivo, and that in vivo high levels of TGF-beta may protect these tumors from cytotoxic cancer therapies.