1. Crude preparations of β-lipoprotein inhibited the fibrinolytic and caseinolytic activity of plasmin: they were contaminated by α 2 -macroglobulin and α 1 -antitrypsin, known inhibitors of plasmin. 2. Purified β-lipoprotein, free from α 2 -macroglobulin and α 1 -antitrypsin, did not inhibit plasmin. It is concluded that β-lipoprotein does not have antiplasmin properties.
1. Purified preparations of α 2 -macroglobulin inhibited the ability of urokinase to induce lysis of a fibrin plate, to activate plasminogen in a purified system, and to hydrolyse N -α-acetyl glycyl-l-lysine methyl ester. 2. Inhibition of urokinase by α 2 -macroglobulin was immediate: no progressive inhibition could be detected using fibrinolytic and esterolytic assays. Inhibition was competitive. 3. The fibrinolytic activity of euglobulin precipitate prepared from exercise plasma was also inhibited by α 2 -macroglobulin.