The effects of exogenously added urokinase type plasminogen activator, tissue type plasminogen activator, plasmin and thrombin on the proliferation of primary cultures of cells derived from prostatic hyperplasia or prostatic carcinomas were investigated by measuring the incorporation of 3H-thymidine into the cultures. Addition of urokinase type plasminogen activator (1.35 x 10(-9) M) or thrombin (10(-7) M) to the culture medium caused a two-fold increase of 3H-thymidine incorporation, regardless of the origin of the prostatic cells. Tissue type plasminogen activator did not alter the rate of 3H-thymidine incorporation, whereas plasmin caused a 25% decrease of 3H-thymidine incorporation in all cultures.