The levels of prostatic acid phosphatase (PAP), gamma-seminoprotein (gamma-Sm) and prostate specific antigen (PA) were determined in the serum of 200 untreated patients 28 patients with reactivated prostatic cancer and 179 patients with benign prostatic hypertrophy (BPH) from 1979 to 1987. PAP and gamma-Sm were determined using an Eiken and Chugai kit, respectively and PA was assayed using an Eiken or Travenol kit. The sensitivity of PAP, gamma-Sm and PA respectively in the untreated prostatic cancer cases was 0, 0% and 67%, for Stage A1, 25, 17 and 100% for Stage A2, 23, 50 and 60% for Stage B, 62, 81 and 94% in Stage C, 58, 67 and 90% for Stage D1, 86, 88 and 100% for Stage D2. The specificity of PAP, gamma-Sm and PA is 89, 69 and 43%, respectively. The efficiency of PAP was the highest at all stages as a whole, but when compared at each stage, gamma-Sm was the highest at Stages B and C. The sensitivity of simultaneous assays of PAP and gamma-Sm was slightly increased, but sensitivity was not increased by simultaneous use of three markers. The efficiency of a simultaneous assay was lower than that of a single assay with PAP. However, combined determination of gamma-Sm or PA with PAP was found to be useful for monitoring the clinical course of the reactivated patients. Correlation between PAP and PA levels was high, but that between gamma-Sm and PA levels was low. There was no correlation between PAP and gamma-Sm levels. In conclusion, PAP is the most valuable marker for prostatic cancer, and gamma-Sm is of value for use in combination with PAP. However, an additional PA assay was not found to be of advantage.