1. Type I antipneumococcus horse serum, in amounts exceeding a characteristic optimum, fails to protect mice against infection with the homologous type pneumococci. This failure is due to a marked inhibition of the phagocytic mechanism in the earlier stages of the infectious process. On the other hand, antipneumococcus rabbit serum in similar quantities does not inhibit phagocytosis, nor does it block the protection.
2. The experimental evidence suggests that the prozoning action of immune horse serum is due primarily to some characteristic property of the specific antibody and secondarily to an heterologous component of the serum, ineffective in itself but acting through the mediation of the antigen-antibody combination. This secondary factor may be a lipid.