The peroxidase-anti-peroxidase technique was used to stain for prostate specific acid phosphatase and prostate specific antigen in 12 patients with primary tumors and in 12 patients with metastases in whom the nature of the tumor was in doubt after routine histopathological studies. Nine of the primary tumors were positive for both markers and an additional 2 tumors stained for prostate specific antigen only. Six metastatic lesions stained for both markers and a seventh for prostate specific antigen alone. Thus, 11 of 12 primary tumors and 7 of 12 metastases studied were proved to be of prostatic orgin. While the peroxidase staining was sometimes weak and uneven this method, using prostate specific antigen and prostate specific acid phosphatase, allowed for ready identification of metastases. The heterogeneity of the tumors in regard to these 2 prostate markers is demonstrated, and the value of staining for prostate specific acid phosphatase and prostate specific antigen is emphasized.