Abstract: The present invention relates to the use of a substance which inhibits the plasma proteins .alpha..sub.2 -AP and/or .alpha..sub.2 -M in the field of the determination and assay of plasminogen activators and inhibitors of said plasminogen activators. It relates in particular to a method of determining (i) a plasminogen activator selected from the group consisting of tissue plasminogen activators (tPAs) and urokinase plasminogen activators (uPAs), and (ii) an inhibitor (PAI) of said plasminogen activator, said method, which involves converting plasminogen to plasmin and then assaying the plasmin resulting from said conversion, comprising inhibition of the plasma proteins .alpha..sub.2 -antiplasmin and/or .alpha..sub.2 -macroglobulin by means of a substance selected from the group consisting of metalloproteinase materials.