α-Tocopherol (vitamin E) inhibited the activity of urokinase on fibrin plates and on the amidolytic substrate S-2444 in a concentration- and time-dependent manner. Inhibition was not removed by dialysis. In the presence of dilutions of plasma, protein-containing fractions of gel-filtered plasma, or bovine serum albumin, the inhibition of urokinase by α-tocopherol was abolished. Urokinase activity was not inhibited by phytol, menadione or hydroquinon