We have generated a monoclonal antibody against prostatic adenocarcinoma, PC-Ab (IgM) was derived from a fusion using the fresh prostatic tissue of adenocarcinoma as the immunogen. Initial screening was performed by enzyme-linked immunosorbent assay (ELISA) on the soluble fraction of the immunogen. The specific analysis was performed by the avidin-biotin-complex immunoperoxidase method on paraffin-embedded sections of normal, benign and malignant neoplastic tissues from the prostate and various organs. PC-Ab reacted with well differentiated adenocarcinoma (83.3%), moderately differentiated adenocarcinoma (92.2%), and poorly differentiated adenocarcinoma (90.2%), respectively. According to classification by stage, the reaction rates with stage T0, T1, T2, T3 and T4 were 77.3%, 80.0%, 95.2%, 91.3% and 92.9%, respectively but no significant differences in the stages were seen among these groups. PC-Ab reacted with the epithelium of the normal prostate and benign prostatic adenoma, and human fetal tissues. Molecular weight and isoelectric point of the antigen recognized by this PC-Ab was estimated to be 57,000 daltons and 7.0, respectively. These results indicate that PC-Ab reacts with the antigen associated with human prostatic adenocarcinoma.