We have investigated about the nature of H-antigen comes from rodent-lung communicated by Rickettsia orientalis and have recognized that the H-antigen shows positive floculation against the homologous antibodies specifically as-follows: 1. The serum comes from guinea pig and rabit floculaes the H-antigen specifically. 2. H-antigen does not appear floculation against the serum prepared from the patients of which are Murine, typhus, Malarie, Leprosy, Scarlet fever, infectious Nephritis, Pneumonia, Tuberculosa.