In a previous paper we reported that an inhibitory substance had been demonstrated in some human sera showing prozone reactions with the antigens of lymphogranuloma venereum and psittacosis (Schmidt et al., 1952). When a second human serum, which ordinarily would fix complement with lymphogranuloma venereum and psittacosis antigens, was allowed to react with these antigens in the presence of the serum showing prozones, complement fixation was inhibited in the region of the prozones. Karrer et al. (1950a) have reported an instance in which a pigeon anti-ornithosis serum showing a prozone reaction displayed similar inhibitory activity. Other investigators have discovered that the immune sera of certain avian and mammalian species will not fix complement with their homologous antigens, but will inhibit the fixation of complement which normally occurs in the presence of the antigen and homologous complementfixing immune sera of other species (Calmette

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