The reactivity of anti-Rickettsiae human antibodies with Proteus OX cells is used as a presumptive rickettsial diseases diagnostic test Weil-Felix reaction. In presented studies we compare the reactivity of human anti-Rickettsiae and rabbit anti-Proteus antibodies with series of Proteus OX2, OX19 and OXK lipopolysaccharides (LPS). Polyclonal rabbit anti-OX2, anti-OX19 and anti-OXK sera reacted only with the homologous LPS--OX2, OX19 and OXK, respectively. The antibodies of Japanese spotted fever patients were less specific and reacted with OX2 as well as OX19 LPS. The antibodies of scrub typhus patients recognized Proteus OXK LPS, only. The serological reactions of O-antigen of P. mirabilis S1959 indicated that this, previously serologically not classified, strain may belong to the OXK group. Bacteria, used in the studies, came from the American, Japanese and European strain collections. The series of OX2, OX19 and OXK LPS, isolated from these Proteus strains, presented pattern of electrophoretic mobility and serological reactivities specific for each of OX-group types.