Monoclonal antibodies were raised to PC-3 human prostate adenocarcinoma cells, and one hybridoma, designated F77-129, was extensively purified and used to characterize a PC-3 antigen. The F77-129 antibody also showed serological reactivity with the Du-145 prostate cancer line and with three of four breast carcinoma lines tested; it showed variable binding to a colon carcinoma line. Several other lines tested, including melanomas, fibrosarcomas, and leukemias, were completely negative. Immunoperoxidase staining of frozen surgical specimens showed binding to both normal and malignant prostate and breast tissue. Injection of radioiodinated F77-129 into tumor-bearing nude mice showed specific in vivo targeting to prostatic cancer implants. The antigen also showed surface modulation by bound antibody, suggesting possible clinical utility of this antibody in delivering immunotoxins to tumors.