The influence of sedimentation temperature upon the hemagglutination-inhibition titer of para-influenza 1 (HA-2) virus was dependent upon the animal species furnishing the erythrocytes. To a degree, higher sedimentation temperatures caused an increase in titer if guinea pig or human type "O" erythrocytes were used, while an opposite thermal effect was exerted upon the agglutination of sheep, rooster and rabbit erythrocytes. Possible explanations of this behavior are discussed. Treatment of serum with receptor-destroying enzyme, incubation of the treated serum and virus for 30 minutes and sedimentation of human type "O" erythrocytes at room temperature were found to be optimal conditions for performances of hemagglutination-inhibition tests. Four hemagglutinating units of the Copenhagen 222 strain, standardized at room temperature, were used. The relationship between virus concentration and the neutralizing capacity of human serum was expressed by neutralization lines. Their slope was 3.8. Neutralization titers were found to be highest with the serum-virus mixture incubated at 35[degree]C for 1 hour before inoculation into monkey-kidney tissue cultures.