1. The effectiveness of various complements in the activation of anti-H. influenzæ horse, rabbit, and guinea pig sera as indicated by bactericidal action has been studied.

  2. The mechanism of the bactericidal action of all these sera involves antibody and complement.

  3. With anti-H. influenzæ horse serum the results of this study indicate that (a) fresh human (infant) serum is effective in complementing this antibody; (b) guinea pig complement is ineffective in this system and such failure is not dependent on the diluent, the virulence of the culture, the age of the culture, the time of incubation of the experiment, or heat-labile anticomplementary bodies in the immune serum; (c) fresh normal rabbit and horse sera do not activate the antibody; (d) fresh immune horse serum in this in vitro system has no bactericidal action; and (e) the ability of the various fresh sera to serve as bactericidal activators of anti-H. influenzæ horse serum is not related to their titer of hemolytic complement for sheep red blood cells sensitized with rabbit hemolysin.

  4. The bactericidal function of anti-H. influenzæ rabbit serum can be activated by human and rabbit complements but not by guinea pig complement; the potency of human and rabbit complements in this respect is essentially the same; and no direct correlation is apparent between the hemolytic titers of guinea pig and either human or rabbit complements and their ability to activate this bactericidal system.

  5. The bactericidal function of anti-H. influenzæ guinea pig serum can be activated by human, rabbit and guinea pig complements. With fresh immune guinea pig serum the quantitative activation of complement is equivalent in the bactericidal and hemolytic systems. The inactivated serum can be fully reactivated by normal guinea pig serum. Fresh human and rabbit sera complement the bactericidal action of immune guinea pig serum to an extent which roughly corresponds to their relative titers in complementing the hemolytic system.

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Aided by the Philip Ellis Stevens, Jr., Memorial Fund.

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