From August, 1981 to May, 1984, we measured gamma-seminoprotein in the serum of 51 untreated patients with prostatic cancer in the Chiba University Hospital. Prostatic acid phosphatase (radioimmunoassay) in serum was also measured in these patients. We also measured gamma-seminoprotein and prostatic acid phosphatase in serum of patients under control by hormonal treatment and of reactivated patients. In untreated stage B and stage C cases, positive rate of gamma-seminoprotein in serum was larger than that of prostatic acid phosphatase. Therefore the measurement of gamma-seminoprotein in serum is considered to be useful in the diagnosis of early prostatic cancer. Four weeks after hormonal treatment, gamma-seminoprotein in the serum of 74% of the patients returned to the normal level. The positive rate of gamma-seminoprotein in the serum of reactivated patients is significantly larger than that of the patients under control by hormonal therapy.