The complement-fixing antigens of the viruses of influenza type A and B present in the allantoic fluid of the chick embryo were studied by centrifugation and adsorption. Two fractions of complement-fixing antigen were found, the one sedimentable at 90,000 g and adsorbable onto particulate components of normal tissues and onto chick red cells, the other non-sedimentable at 90,000 g and non-adsorbable. The ratio of concentration of sedimentable to non-sedimentable fraction decreased with time of incubation of the infected embryos.

The 2 fractions either from influenza A or B virus differed in their optimal antibody relationships, in that maximal serum titer for the sedimentable and adsorbable material was approximately 4 times higher than for the non-sedimentable and non-adsorbable fraction.

Absorption of influenza A convalescent serum with the 2 antigenic fractions revealed the presence of 2 serologically distinct antigens in the sedimentable material. One of these, the more active antigen, was identical with the non-sedimentable fraction. In addition, some Forssman antigen was found in the sediment which, however, did not interfere in the reactions.

The activity of the antigen which occurred only in the sedimentable fraction was relatively low and did not account for the difference in optimal serum titers encountered with the sediment or the supernatant fluid as antigens. Other possible explanations for this phenomenon are discussed.

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This study has been aided by a grant from the Medical-Research Division of Sharp and Dohme Inc.

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