Using chick blood cells, the author investigated the Hemagglutination with the supernatants of the emulsized organs of hamsters infected with rickettsia, and obtained following results; 1) The supernatants of emulsized organs of hamsters inoculated with Rikettsia prowazeki showed the maximum hemagglutination titer of 259X eight to ten days after inoculation, with those of rickettsiae mooseri showed the maximum titer of I25X six to seven days after inoculation. 2) The antigen with the highest hemagglutiutination antigenicity was gained from the livrs of the acme of infection. Its hemagglutination antigenicity and the development of the disease showed the parallel relationship. 3) This antigens cause the strongest hemagglutination at pH 7.0 to 7.5 and at the low temperature of 5° fo 10℃ . 4) The hemagglutination by the antigens of Rickettsiae prowazeki and mooseri was inhibited by each antirikettsial rabbit serum and showed fairly high specificity. 5) The inhibition action on hemagglutination by the sera of guinea pigs infected with rickettsia prowazeki and mooseri runs parallel with the development of the disease, especially with fever of the infected guinea pigs.