An improved procedure for measurement of hemagglutinin titer of influenza virus has been developed. Interference due to inhibitors was minimized through the use of a longer period of incubation of the virus-red blood cell mixtures; that due to enhancing agents, by incorporation of gelatin into the diluent medium. Protective action against loss of hemagglutinin was also afforded by the gelatin. The kinetics of the hemagglutinin titration, as revealed by the relationships between long virus

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... on and percentage of transmission of the mixtures of virus and red cells, are described for different strains of the virus, and the implications of the findings with regard to meaningful application of the method are discussed.