Abstract
Attempts were made to detect serologic specificities involved in the serum bactericidal (BC) reaction with Haemophilus paragallinarum (Hpg). On the basis of serum BC reactions, ten Hpg isolates were classified into two distinct serotypes. Specificity of the BC activity toward the encapsulated organisms of Hpg defined by the BC assay is similar to serotype specificity defined by the hemagglutination-inhibition or by the agglutination tests.