Changes in the capacity of fibrinogen subjected to oxidative modification to transform into fibrin under the effect of thrombin and to form a fibrin clot were studied. The effects of oxidized fibrinogen preparations on the clot formation by citrate-treated donor plasma were evaluated by the thrombin time test. Oxidation impaired the capacity of isolated fibrinogen to form a fibrin clot under the effect of thrombin. Addition of oxidized fibrinogen solutions to donor plasma led to prolongation of the plasma clotting time. Maximum addition (33% volume) of oxidized fibrinogen led to a 10-26% prolongation of clotting time in comparison with addition of the same volume of the same solution without fibrinogen.