The secretion of plasminogen activator by explants of 92 human malignant tumors was studied in short-term organ culture. Where possible, adjacent normal tissue of the presumed origin of the tumors was also studied. The study included adenocarcinomas of the lung, colon, prostate, breast and stomach and different types of sarcomas. All tissues were quantitatively extracted to determine the amount of cell-bound enzyme. Both culture fluids and extracts were analyzed with respect to the type of plasminogen activator they contained by immunoinhibition with goat IgG formed against purified human urinary urokinase sodium dodecyl sulfate:gel electrophoresis followed by zymography. The measurement of plasminogen activator secretion rates gives a much sharper differentiation between malignant and normal tissues than does the amount of extractable enzyme. The enzymes secreted in short-term organ culture are, in the great majority of the cases studied, of the urokinase type, even when a large fraction of the activator contained in the tissue is of the vascular type. The secretion rates of metastatic tumors of the colon are much lower than those of the primary ones. Immunoperoxidase staining of tissue sections reveals that urokinase is localized predominantly in the tumor cells. The low secretion rates of metastatic tumors, probably a reflection of this property in the original cell that gave rise to the metastatic focus, could be of advantage to circulating cancer cells. Such cells would not dissolve the microthrombus thought to be essential for the arrest of cancer cells in the capillaries of target organs.