Hemagglutination inhibition (HI) tests with parainfluenza antigens were made on patients, chiefly adults, with acute respiratory illness.
1) Four-fold or greater monotypic rise in HI titer was obtained in 0.6, 0.7 and 4.1 per cent of the patients for types 1 (HA-2), 2 and 3 of parainfluenza virus, respectively. The serologic diagnosis could not be made with certainty in 3 cases because of cross-reactions.
2) Those cases showed upper-respiratory inflammation, bronchitis, pneumonitis or influenza-like illness and varied in severity.