Abstract: Cobra Venom Factor (CVF),an anticomplement factor,had been separated from the venom of Chinese cobra,Naja naja atra.The anticomplementary activity and the hemolytic activity of CVF were observed by methods in vitro.Complement C3 was activated and depleted by CVF through the alternative pathway leading to the decrease of the total complement activity.Cross immunoelectrophoresis showed that antigenicity of C3 had changed after C had been cleaved by CVF.When guinea pigs were injected I.p.with purified CVF about 0.15 μg per grm body weight,the hemolytic complement activity of guinea pig serum reduced to less than 3% of normal level within 24 hours and recovered completely after 13 days.No toxic effect of CVF was observed.On rocket immunoelectrophoresis analysis,CVF and its subunits could not cross react with antiserum to human C3,but normal human serum could cross react weakly with antiserum to CVF.Amino acid composition of CVF was found to be similar to human C3 and its physiological cleavage products.The rocket immunoelectrophoresis analysis showed that some kinds of venom of different snake-species,I.e.for of family Elapidae (Naja naja atra),Ophiophagus hannah,Bungarus multicinctus,B.jasciatus,one of Viperinae (Vipera russelli) and two of Hydrophiidae (Hydrophis cyanocinctus,Lapemis hardwickii) had cross reactivity with anti-CVF to some extent,however,these venoms (except the venom of) Naja naja atra had no anticomplementary activity.Venoms of two species of Crotalinae (Agkistrodon acutus,A.halys) did not show such cross-reacting,but they had strong anticomplementary activity.