Inhibitors present in normal human and animal sera prevented hemagglutination by and neutralized infectivity of inhibitor-sensitive influenza A2 virus. Starch zone electrophoresis of sera indicated that the same serum components possess both hemagglutination-inhibiting and neutralizing activities. The greatest amount of inhibitory activity was found in normal horse serum, and the inhibitory activity increased with heating or treatment with concentrated solutions of urea. The inhibitory activities on human, ferret, and rabbit sera were markedly reduced but not completely eliminated by V. cholerae filtrate and purified neuraminidase. The inhibitory activity of horse serum was only moderately reduced by these agents. The nature of the horse serum inhibitor and the differences in the interactions of inhibitor-sensitive and insensitive influenza A2 virus particles and pre-1957 influenza viruses with receptors have been discussed.

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