In a serological study of 16 strains of epidemic typhus and 3 strains of endemic typhus by complement-fixation tests in which titrations of guinea pig immune serums were made against suitable predetermined dilutions of antigen, it has been shown that serological homogeneity exists among all the epidemic strains and also among the 3 endemic typhus strains tested. Though a common antigenic factor is present, differentiation between epidemic and endemic typhus is indicated by higher titers against the homologous strain than against the heterologous strain.