The introduction of immunoperoxidase and the indirect immunoperoxidase technique made important contributions in histopathologic diagnosis of prostatic cancer. This staining can be performed on formalin-fixed paraffin-embedded tissue which is usually available. We have used this histopathologic staining technique in 56 patients. The tissues include primary and metastatic prostatic cancer tissue in addition to normal renal pelvis and bladder tissue from other patients. Our data indicate that acid phosphatase can be localized in prostatic cells but not in transitional cells. Therefore, immunohistochemical staining of prostatic acid phosphatase seems most useful to identify metastatic prostate adenocarcinoma or primary tumor and to differentiate them from intraductal prostatic transitional carcinoma or other transitional cell carcinomas.