SummaryThe plasmin substrate, H-D-norleucyl-hexahydrotyrosyl lysine-p-nitroanilide (Spectrozyme-PL), was found to be equiva lent to 6-aminohexanoate as an enhancer of porcine and human plasminogen activation by urokinase and of removal of the 1-77 peptide of plasminogen by plasmin. Activation of plasminogen lacking kringles 1-4, on the other hand, was not influenced by Spectrozyme PL. Although the rate of activation of human plasminogen and the modification of human plasminogen by plasmin are faster by an order of magnitude than that of the activation and modification of porcine plasminogen, both reactions in the human zymogen, the hydrolysis at arg561-val562 and at lys77lys78, are accelerated by Spectrozyme PL. The findings indicate that kinetic interpretation of plasminogen activation in solutions containing substrates, where the substrate has been incorporated to inhibit feedback proteolysis by plasmin, must account for the cofactor activity as well as the inhibitory activity of the substrate.