The author shows in this paper the results of the inactivation of com¬plement or alexin by some Brazilian snakes venoms of the Elapideae and Cro¬talideae families. The venom of Micrurus frontalis (Elapideae family) and of Crotalus terrificus (Crotalideae family) did not destroy the complement; but the venoms of Bothrops generus (B. atrox, B. neuwiedii, B. jararaca, B. jararacussú, B. cotiara e B. alternata), (Crotalideae family) are highly active in the inactivation complement, always destroing the fourth component, whichis exactly the same component that is detroyed by ammonia.