The reactivity of monoclonal antibody (MAb) antileu 7 (HNK - I) on formalin-fixed sections of prostate tissues was determined by an immunoperoxidase assay. Anti-Leu 7 was found to react with both primary and metastatic tissues from 6/6 normal prostate, 15/15 benign prostatic hyperplasia, and 37/39 prostate carcinoma samples. Anti-Leu-7 reactivity was localized in the cytoplasm of the supranuclear region of prostatic epithelial cells and the fibromuscular stroma did not stain. Myelinated nerve fibers in the stroma were stained with anti-Leu 7 and this served as an internal control. Anti-Leu 7 did not bind to prostatic acid phosphatase (PAP) or prostate-specific antigen (PSA) in direct or competitive binding radioimmunoassays. Anti-Leu 7 was more effective (5/5) in the identification of metastatic tumors of prostate origin than an MAb to PSA (2/5). The differential pattern of reactivity of anti-Leu 7 compared to anti-PSA on serial sections of prostate tissues suggests that Leu 7 may be a useful diagnostic and prognostic marker of prostate cancer.