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We have carried out enzymatic, immunofluorescence, and surface iodination studies which show that B16 melanoma cells express the single chain form of the urokinase type plasminogen activator (uPA) on their cell surface, and that these cells are capable of plasminogen-dependent fibronectin degradation. The significance of the expression of surface single-chain uPA and uPA activity to the metastatic process was examined by preincubating melanoma cells with uPA modulating agents followed by i.v. injection of the cells into mice and enumeration of pulmonary nodules 17 days later. B16 cells that had been pretreated with anti-uPA immunoglobulins that were inhibitory to uPA activity invariably showed significantly decreased numbers of metastases compared to controls. On the contrary, pretreatment with plasmin, which is not only the product of the uPA catalyzed reaction but is also able to convert single-chain …