Under fibrinolytic therapy, the levels of fibrinogen and of fibrin(ogen)degradation products (FDP) grossly influence most of the laboratory tests usually performed for clinical control of fibrinolytic treatment. Although these interferences are basically known, there is a lack of more detailed analysis under true clinical conditions. Therefore, fibrinogen determination according to Clauss (1957), thrombin time, and thromboplastin time were used for routine monitoring of 29 patients undergoing therapy with streptokinase for up to 6 days, and the results of these tests were correlated with true clottable fibrinogen as determined according to Ratnoff and Menzie (1961) and with FDP as determined according to Merskey et al. (1966).Fibrinogen determination by the Clauss method yielded poor results; for immediate clinical control, however, they were sufficient, when values above 80 mg/dl were obtained (r=0.60 when compared to actual fibrinogen concentration). Values below 80 mg/dl were completely unreliable and more closely reflected FDP (r=0.63) than actual fibrinogen levels (r<0.20). The Clauss assay is entirely insufficient, when exact fibrinogen determinations are required as for rheological or pharmacological considerations. Quite in contrast, fibrinogen levels measured according to Ratnoff and Menzie were independent of FDP levels.After bilogarithmic transformation, the thrombin time closely correlated to the FDP level (r=0.86); it thus is a valuable test for quickly estimating FDP. Since FDP also greatly influence thromboplastin time determinations (r=0.79), they must duely be considered, when overlapping oral anticoagulation is planned towards the end of fibrinolytic therapy.