The plasminogen activator content of surgically removed lung cancers, as well as that of adjacent normal lung tissue has been determined quantitatively. Optimum conditions for the quantitative extraction have been worked out using a modification of the technique described by Nagy et al. (Int. J. Cancer, 19:614-620, 1977) which utilizes a buffered solution of the non-ionic detergent Triton X-100. Plasminogen activator was significantly elevated in the tumors (2.5- to 4.3-fold over the normal lung, depending on sample selection and other criteria), although individual variations between tumors were extremely large. No significant correlation was found between the histopathological character of the tumors and the activator content, or between invasiveness and activator content. Using rabbit antibody formed against purified urokinase as an inhibitor of activator action, it was found that lung tumors contain a urokinase-like enzyme as the predominant plasminogen activator, while the activator content of the adjacent normal lung tissue consists of some urokinase-like enzyme, but mostly of an enzyme which is not inhibited by the antibody. The urokinase-like activator has been purified approximately 20,000-fold from lung tumors by the combination of two affinity chromatographic procedures, and was compared with purified urokinase with a molecular weight of 55,000 on all criteria used, the lung tumor activator was identical to urokinase.