A zymographic technique for the detection of inhibitors of plasminogen activators has been applied to the analysis of plasma. Two inhibitors have been demonstrated; one inhibited plasmin with consequent apparent inhibition of urokinase and tissue-type plasminogen activator activity; it was identified as alpha 2-antiplasmin. The second inhibitor, as yet unidentified, had a molecular weight of about 40K. It inhibited urokinase and tissue-type plasminogen activator but had no effect on plasmin.