The influence of heparin therapeutic concentrations (0.25-10 U/ml) on the kinetic indices of human serum complement activity has been studied. An acceleration of complement-dependent lysis of rabbit erythrocytes via alternative complement pathway was found at low (0.25-0.5 U/ml) heparin concentrations and complement inhibition at higher (5-10 U/ml) concentration. The alternative pathway inhibition was revealed mainly by lag-period elongation of the hemolysis. Heparin in the concentrations more then 1 U/ml caused dose-dependent lowering of sheep erythrocytes hemolysis rate accompanied with reciprocal elevation of complement hemolytical capacity, which shows more "economical" consumption of complement components at heparin presence. The identical effect was observed in sera, deficient in factor D of alternative pathway of complement activation. This can be an evidence of the prevailing heparin inhibition of the classical pathway.