The properties of fibrinogen and fibrin, the levels of fibrincpeptide A (FPA) and fibrin(ogen) degradation products (FDP) were studied in 34 patients with AMI who were undergoing thrombolytic and heparin therapy. They were classified into 6 groups accordingto their stage of treatment: group 1, before intravenous administration of 800.000 U streptokinase over 30 min; group 2, after a<Mnistraticn of SK but before adninistraticn of heparin; group 3, during 24 h ofthe 5 ng/h heparin continuous infusion; group 4, during 48-72 h of the 16.6 ng/h heparin continuous infhsion; group 5, after 1 week of administration of SK and with a bolus inyection of 50 rg heparin every 4 h; group 6, patients who were undergoing only heparin treatment. The Fg 1/ Fg II ratio varies during treatment with SK and heparin. In group 1 a sligjnt increase (2.5) is observed. Group 2 shows a significantdecrease (0.6) as a result of fibrinolysis. In group3 the ratio reaches normal value (1.8) while in the fourth group it is twice the normal value (4). The value for group 5 is nearly normal (2.1), and in group 6 it reaches values similar to those obtained in group 4, which implies that the rise in the Fgl/Fgll ratio is not a result of fibrinolytic treatment. TheFPA level shows and increase in patients with AMI (group 1,126 ng/ml). When SK treatment is applied (group 2), FPA decreases to 52 ng/ml. Later treatment with heparin (group-3, 82; group-4, 44 and group-5, 81ng/ml) does not neutralize thrcmbinic activity. Patients treated only with heparin (group 6) show an FPAvalue of 19 ng/ml, which is lower than in the other groups. All of this indicates that thrombin is activated after fibrinolytic treatment. FDP values show asignificant increase in the six groups (1, 53; 2, 430; 3, 128; 4, 270; 5, 139 and 6, 141 ug/ml), which indicates that during treatment with heparin the fibrinolytic activity persists. he formation of highly cross-linked fibrin is altered in groups 1,2,3 and 4,as a consequence of circulating FDP effect and fibrincgeno- lysis.The permeability of the fibrin clotdecreases in groups 1 (0.42), 2 (1.3), 4 (1.1) and 5(0.5 ml/s/ng) and increases in group 2 (23.2 ml/s/nig) with respect to the normal plasma value (3.2 ml/s/nrg). The decrease in permeability must be related to the existence of hypercoagulability resistant to heparinization. FPA values, tine Fgl/Fgll ratio, andfibrin permeability can be used to evaluate the degree of thrcmbin activity during thrombolytic treatmentinAMI.