The results of mouse protection tests with 136 human sera and a strain of human influenza virus are described. After the 1st year of life, the sera of approximately half the individuals tested contained sufficient antibody to furnish complete protection to mice. A much higher percentage of the sera obtained from individuals recently convalescent from influenza exerted a completely protective effect. On the other hand, certain sera protected only partially under the conditions of the tests.

The results have been compared with those obtained by Shope in tests done with the same sera against swine influenza virus. The possible epidemiological significance of the results is discussed.

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