Antistreptokinase IgG (antiSK IgG) from blood of 8 patients with acute myocardial infarction that were treated by streptokinase (SK) has been investigated. AntiSK IgG contained 1.8% of total serum IgG. They had high affinity to SK (K50% approximately 10 nM) and inhibited activation of plasminogen (Pg) by SK with K50% approximately 6 nM. AntiSK IgG were bound with chymothriptic fragments of SK with affinity decreased in the set of fragments: 47 > 36 > 30 > 17 > or = 11 > 7 kDa. 11 linear epitopes of antiSK IgG were localized in I1-S12, T43-M70, G139-Q152, T163-I190, T193-S222, F241-Y252, Y275-P286, T315-L336, I365-E376, S379-T390 and Y397-N410 sites of SK primary structure using SK decapeptides. 70% of antibodies were bound with T43-M70 (38.3%), T315-L336 (13.2%) and Y397-N410 (17.7%) SK sequences located in alpha and gamma SK domains. By depletion of antiSK IgG on Pg-SK complex it was shown that 80-85% of antiSK IgG bound to Pg-SK complex, 47.9% of that contained antibodies with epitopes located in I1-S12, T43-M70, T193-S222 and S379-T390 SK sequences, and rest of IgG had probably spatial epitopes. Unbound with Pg-SK complex antibodies inhibited activation of Pg by SK with higher affinity (Ki approximately 1.2 nM) in comparison with total antiSK IgG fraction. The role of different sites in antigenity of SK and in Pg-SK complex formation is discussed.