SUMMARY: Normal mouse serum contains a haemagglutination inhibitor and a neutralizing factor for influenza virus. The inhibitor and the neutralizing factor are heat-labile, can be destroyed by a crude filtrate of and are much more active towards unadapted virus than towards mouse-adapted virus. The inhibitor enters into stable combination with the virus in the presence of Ca ions. It is adsorbed by a large amount of unadapted virus, even after the haemagglutinin of the latter has been destroyed by heating at 60° for 15 min. Three strains of freshly isolated influenza B virus tested were not significantly neutralized by normal mouse serum. By growing two strains of unadapted influenza A virus in eggs in the presence of normal mouse serum, two variants which are resistant to the neutralizing action of mouse serum were produced. It appears that this type of variation also occurs when an unadapted virus is passed in mice. The theoretical implication of this type of variation in response to a normal host component and its relationship to the mouse-pathogenicity of influenza virus are discussed.


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