The fixation of guinea pig complement by aggregates of rabbit antibody against ovalbumin and ovalbumin is impeded by normal rabbit serum. The inhibitory effect of rabbit serum is a function of the γ-globulin fraction. Prolonged storage in the cold, or heating at 56–65°C for 1 hr or less have no significant effect on this activity of rabbit serum or its γ-globulin; exposure to 85°C results in the loss of activity within 30 min. The inhibitor can be removed from serum by absorption with bentonite, celite or Zymosan, or by “decomplementation” with large doses of specific precipitate. Preliminary kinetic studies show that the inhibitor retards the rate of complement fixation and that its action can be overcome, in part or entirely, by suitable extension of the incubation period. The mode of action of the inhibitor has been discussed in terms of two hypotheses, namely: a) the possibility that the inhibitor combines with the antigen-antibody aggregates in a manner which blocks fixation of complement, or b) that it combines reversibly with complement.