The serum prostate specific antigen (PA) was determined with the Diagnostic Products Cooperation (DPC) PSA double antibody radioimmunoassay kit. The upper limit of the normal range was set at 4 ng/ml which was the mean + 3S.D. for males over 50 years old in a mass examination. For comparison, prostatic acid phosphatase (PAP), and gamma-seminoprotein (gamma-Sm) were determined using an Eiken kit and Chugai kit, and PA was also assayed using another kit (Eiken, Travenol). Positive rate of PA and PAP in the untreated prostatic cancer was 75 and 33% in Stage A, 100 and 0% in Stage B, 100 and 100% in Stage C, 100 and 67% in Stage D1, 100 and 80% in Stage D2 and 73 and 33% in benign prostatic hypertrophy (BPH), respectively. The level of PA determined during the follow-up of prostatic cancer showed the usefulness of simultaneous PA and PAP assays for monitoring the clinical course. The PA level using a DPC kit was highly correlated to that of PA using other kit, but the correlation with gamma-Sm and PAP was low. These results show that the DPC kit is useful for determining PA, and determination of PA and PAP is of great value both in diagnosis and in the follow-up of prostatic cancer, but the high positive rate in BPH remains a problem.