Serum prostatic specific antigen (PSA) and prostatic acid phosphatase (PAP) were evaluated with double monoclonal radioimmunoassays. In 250 patients with prostatic cancer the normal limits were as follows: PSA 0.1-2.7 ng/ml, and PAP 1.09 +/- 0.45 ng/ml (mean +/- SD). In 91 untreated patients with non-metastatic tumours, 42.8% had PSA greater than 10 ng/ml and 18.6% had PAP greater than 2 ng/ml. In 60 untreated patients with metastatic disease PSA was greater than 10 ng/ml in 91.7%; PAP was greater than 2 ng/ml in 65%. In prolonged remission PSA was generally less than 5 ng/ml and PAP less than 2 ng/ml. Longitudinal studies of 2-4 years showed the independence of these markers and a higher correlation of changes in the PSA level and clinical status than given by parallel PAP measurements. In non-metastatic disease, PSA greater than 10 ng/ml at presentation, with or without a coincidentally raised PAP, carried an increased risk of progression within 2 years.