Plasminogen activators (PA), particularly the lower Mr urokinase (u‐PA) type, have been associated with tumor cell invasion and metastasis. We have examined the expression of PA by two human prostate cancer cell lines (PC‐3 and DU‐145) using functional and immunologic techniques. The culture media and cell extracts of the more aggressive PC‐3 cell line contained more than two‐fold greater PA activity than the relatively indolent DU‐145 cell line. Zymographic studies identified the PA expressed as u‐PA. PC‐3 cells expressed an additional lower molecular weight form of u‐PA not noted in DU‐145 cells. Heterogeneity in u‐PA expression was shown by the fibrin lysis assay, immunohistochemistry, and dual parameter flow cytometry indicating the presence of phenotypically divergent cell populations. Increased u‐PA expression may identify those tumor cells that possess aggressive biological potential.