Tissue specimens from 12 patients with adenocarcinoma of the prostate and 7 patients with benign prostate hypertrophy were stained by an indirect immunoperoxidase method using antiproliferating cell nuclear antigen (PCNA) monoclonal antibody. The PCNA labeling index was determined by counting the number of PCNA-labeled cells in the tissue sections. Average PCNA labeling index of the benign prostate hypertrophy was 1.2 +/- 0.5%. Poorly differentiated tumors averaged 7.6 +/- 3.9% labeling versus 4.6 +/- 1.3% in moderately differentiated tumors, and well differentiated tumor in the series had a PCNA labeling index of 2.5 +/- 0.9%. The PCNA labeling indices for atypical hyperplasia were 1.9, and 4.1%, respectively. Our preliminary results suggest that the measurement of PCNA labeling index in prostate cancer may prove to be a new objective and quantitative assay of biological potential of individual tumor.