Serums from guinea pigs inoculated with different strains of tsutsugamushi disease (scrub typhus) and serums from 5 cases of the disease accidentally infected with known strains, when tested by complement fixation, yielded markedly higher titers with the homologous strain than with heterologous strains. Infections with 2 strains in particular, the Karp and Gilliam strains, were clearly differentiated and, therefore, it is desirable that both antigens be employed in testing serums for diagnosis from cases of suspected illness.