Three types of htmagglutinin showing different biologic and hemagglutinating properties, designated as HA-L, HA-HL, and HA-HS, were found in serotype 1 strains of Haemophilus paragallinarum. Serologic and immunologic differences in hemagglutination-inhibition (HI) antibody-producing antigenicity and in protective activity were observed among them. It was observed that protective immunity was induced in injection only with HA-L hemagglutinin as an immunogen. Protected chicks had HI antibodies against both antigens HA-L and HA-HL. Regardless of having HI antibody against HA-HL antigen, protective immunity was not induced in chicks by injection with HA-HL hemagglutinin as an immunogen. The HA-HS hemagglutinin lacked either HI antibody-producing antigenicity against antigens HA-L and HA-HL, or protective activity, but nonspecific HI antibody against HA-HS antigen was detected even in uninjected control rabbit sera. Protective immunity of the HA-L hemagglutinin was completely inactivated by heating at 121°C for 2 hr. When the HA-L hemagglutinins were heat-treated at 72°C to 100°C for 30 min, protective activity remained in a varing extents in the heat-treated hemagglutinins, but HI antibody-producing antigenicity of thus heat-treated hemagglutinins against HA-L antigen was completely lost by the heating. The results suggested that HA-L hemagglutinin is only responsible for immunogenicity among the 3 types of hemagglutinin, but HI antibody formation against HA-L antigen is not always essential to the protection.