1.Complement fixation tests were carried out in a group of 146 pigeons.
2.Positive reactions were obtained in all pigeons with a microscopically diagnosed H. columbae infection and in the majority of latent chronic infections. The complement fixation tests particularly with P. gallinaceum antigen was found to be more reliable than direct blood examination for the diagnosis of H. columbae.
3.Antigens prepared from P. gallinaceum and H. columbae gave specific and group reactions.
4.The existence of serological cross reactions between the genus Plasmodium and the genus Haemoproteus was demonstrated.
5.These results are significant in the light of recent findings of primary tissue forms in Plasmodium.