Using numerous strains of Asian influenza virus the authors were able to demonstrate rises in antibody titer by the hemagglutination-inhibition test for all cases showing a rise with the complement-fixation test. Cross hemagglutination inhibition tests using 20 paired sera and 12 virus strains showed some viruses could detect rises in titers in some sera but not others. Differences in antigenic efficiency or previous immunological response of the patient does not seem to be the explanation.