In this study, Tanizawa, rat IV, rat XXII strains, which were isolated in Kagawa prefecture, and the hitherto isolated Shichito and Karp strains were used. Each fraction was prepared from the brain, spleen, lung and wash of abdominal cavity of infected mouse. The results are as follows: 1) When the brain, spleen, and wash of abdominal cavity are used as the materials, the fresh ones within 5 hours after the preparation cause the remarkable hemagglutination. The same materials, however, when treated with the same volume of ether for 2 hours or heated at 60°C for 30 minutes, lose their hemagglutination activity. The aged materials, left for 7 to 50 days, also lose their hemagglutination activity. 2) The coarse and R fractions prepared from the lung of nasally infected mouse, as far as they are fresh, cause very remarkable hemagglutination, but they lose their activity by treating with ether for 2 hours or by aging. 3) The F and H fractions of the infected lung have no hemagglutination activity. 4) In short, the activity to cause fowl hemagglutination is not present in the rickettsia-infected tissue, but in rickettsia itself. The fact that the aged or ether- or heattreated rickettsia shows no hemagglutination suggests that the hemagglutination activity is attributable to the ether- or heat-labile factor which is not related to the sero-immunological reaction.