Clone B559 mouse melanoma cells are highly tumorigenic and produce plasminogen activator. Cells of clone C3471, a line obtained by continued growth of B559 cells in medium containing 5-bromodeoxyuridine (1 microgram/ml), have no plasminogen activator and are non-tumorigenic. When B559 cells are co-cultivated with C3471 cells, the ability of B559 cells to activate plasminogen is suppressed. Under these conditions cell fusion occurs. Lack of expression of plasminogen activators is not a consequence of cell fusion, inhibition of cell division or release of soluble inhibitors of either plasminogen activators or plasmin. No inhibitors of plasminogen activators could be demonstrated in association with sub cellular fractions of C3471 cells or with the C-type viral particles released from C3471 cells. Close contact between cells of the two lines is shown to be essential for suppression of plasminogen activation.