Controlled extracellular proteolysis is catalyzed in part by the secretion of plasminogen activators. As a step in the study of the expression of these enzymes in mouse tissues, we have isolated five cDNAs encoding the mouse urokinase-type plasminogen activator from a cDNA library prepared with size-selected mRNA from MSV-transformed 3T3 cells. The longest cDNA insert contains the entire coding region of mouse urokinase, 58 base pairs of the 5' non-coding region, and the entire 3' non-coding region, which is 942 base pairs long. The deduced protein sequence, which starts with a signal peptide of 20 amino acids, shows extensive homology to that of human and porcine urokinase. However, in contrast to these enzymes, mouse urokinase contains no N-glycosylation site. Bacteria harbouring one of the recombinant plasmids synthesize and secrete into their periplasm a protease indistinguishable from mouse urokinase.