The reactivity of the anti-Leu 7 monoclonal antibody (Leu 7) was tested on 83 human tumours and on non-neoplastic prostatic, hepatic and pancreatic tissues. A four-step peroxidase-anti-peroxidase method was used on paraffin embedded tissues and we observed strong cytoplasmic positivity in all 19 primary prostatic tumours, in two metastatic, poorly differentiated prostatic adenocarcinomas, and in normal and hypertrophic prostatic epithelium. All the primary prostatic tumours also stained positively for prostate-specific antigen and for prostatic acid phosphatase using polyclonal antisera. The degree of positivity for these antigens varied from case to case. Adenocarcinomas arising from the gastrointestinal tract, pancreas and gallbladder were anti-Leu 7 negative. Focal Leu 7 positivity, largely confined to cell membranes, was observed in some ovarian, endometrial, renal, lung and breast adenocarcinomas. These tumours, as well as some of the gastrointestinal, hepatic and pancreatic tumours, also showed focal cytoplasmic positivity for prostate-specific antigen and prostatic acid phosphatase. Our findings suggest that the anti-Leu 7 monoclonal antibody is a marker that may facilitate the detection of metastatic prostatic adenocarcinoma, especially when used in conjunction with staining for prostate-specific antigen.