An immunoperoxidase technique for prostate-specific antigen was used to evaluate autopsy material from 8 patients with multiple prostatic metastases, 22 primary prostatic carcinomas and 12 nonprostatic tumors. All specimens of primary and metastatic prostatic carcinomas stained positively, irrespective of the Gleason grade. None of the nonprostatic tumors showed immunostaining. The method appeared to be highly specific and highly sensitive for the detection of the prostatic origin of metastases.