It has been shown that haemolysis of sheep erythrocytes by antibody and either guinea pig or human complement is inhibited by salicylaldoxime (0-hydroxybenzaldoxime.) Salicylaldoxime differs in its action on immune haemolysis from such previously described inhibitors as ethylenediaminetetraacetic acid and diisopropylfluorophosphate in that it specifically inhibits the reaction between the intermediate complex EAC′1,4,2 and C′3. The inhibition does not appear to be reversible by divalent cations. To be effective the inhibitor must be present during the course of the reaction, since haemolysis proceeds unimpaired if the inhibitor is added to complement and removed by dialysis, or if added to either red cells or EAC′1,4,2 and removed by centrifugation. The experimental evidence suggests that salicylaldoxime acts by preventing the effective combination of C′3 with EAC′1,4,2, and not by the destruction of C′3.