A simplified procedure, based upon Hirst's test for titrating hemagglutinating capacity of influenza-virus and the corresponding antibody, has been described. The chief modifications involve reduction in the quantity of cells employed, determination of the endpoint of agglutination and inhibition by the pattern of the sedimented cells, and less rigid adherence to a specific time-interval.

The influences of temperature, time, concentration of erythrocytes and order of addition of reagents have been investigated.

The method has been standardized for optimal sensitivity and simple criteria have been adopted for measuring agglutination. The test for antibody-content of a serum has been simplified so as to reduce the amount of material required and by preparation of dilutions of serum directly in the suspension of red cells, thus involving the addition of one rather than two reagents to the serially diluted serum.

The desirability of adopting some uniform scale for expressing the titer of antihemagglutinins in a serum has been discussed.

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