It has been demonstrated that nitrogen which can fix to specific precipitates persists in rabbit, human, and guinea pig sera for long periods of time. The reasons for regarding this as complement nitrogen are discussed. The persistence of complement nitrogen is greatest in rabbit serum. Only rabbit complement affected the precipitation of soluble rabbit antigen-antibody complexes. It has been postulated that the effect of an anticomplementary substance present in sera decreases with age, dilution, and increase in temperature. The effect of versene on the contribution of complement nitrogen to specific precipitates by guinea pig and some human sera has been confirmed. However, versene in the concentrations used had no effect on the complement of rabbit serum.