An improved method is described for producing in guinea pigs sera which contain antibodies only to the soluble (S) antigens of type A influenza viruses and none to the viral (V) antigens. The critical modification was the introduction of S antigen derived from A/equi to serve as the booster injection to recall anti-S which was initially stimulated by active Swine/Cam virus. Since A/equi shares no V antigen relationship with either swine or human influenza strains, concomitant recall of anti-V in animals in which this antibody had already disappeared, by any undetectable V components remaining in the S antigen, was avoided.

The anti-S sera thus produced reacted the same in the complement fixation test, whether titrated against S antigens derived from A/equi, S/Cam or three prototype human strains. Similarly, no strain-specific differences could be demonstrated among the S antigens prepared from these five type A viruses.

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