The pathogenicity and presence of serovar-specific hemagglutinating (HA) antigens of Haemophilus paragallinarum serovar B reference strains 0222 and Spross and field isolates 24268 and 24317 were investigated. Chickens challenged with all strains except strain 0222 showed clinical signs of infectious coryza. For all four strains, challenged chickens had intrasinus lesions and were colonized by the challenge organism. Before and after hyaluronidase treatment, strains 0222, 24268, and 24317 showed HA activity against formaldehyde-fixed chicken erythrocytes but not against fresh chicken erythrocytes. Strain Spross expressed HA activity only after treatment. In cross-hemagglutination-inhibition tests, the four serovar B strains cross-reacted with each other but not with serovar A and C strains. Cross-adsorption tests indicated that strain 24317 has a wider range of HA antigens than the other two strains. Our results indicated that H. paragallinarum serovar B strains are pathogenic for chickens and that they possess six different HA antigens, one of which is specific for serovar B strains.