In mouse MSV-3T3 cells the synthesis of the urokinase form of plasminogen activator was increased 10-fold after addition of the tumor promoter phorbol-12-myristate-13-acetate (PMA). PMA also stimulated the secretion of the protein into the culture medium, mostly in the form of enzymatically inactive pro-urokinase. When assayed by injecting RNA into Xenopus laevis oocytes, the concentration of functional urokinase mRNA was found to be 6- to 10-fold higher in the PMA-treated cells; a similar increase in urokinase mRNA content was measured by hybridisation with a mouse urokinase cDNA probe. Thus, the induction of plasminogen activator by PMA in MSV-3T3 cells is accounted for by an increased content of urokinase mRNA.