Of the sera which were positive to Rickettsia tsutsugamushi by indirect immunoperoxidase test, approximately 80% sera were positive to a Proteus OXK antigen by Weil-Felix test at 10 or more days after the onset of fever, while only 10% sera were positive within 9 days from the onset of fever. In ELISA using the OXK antigen, almost all of the paired sera of tsutsugamushi disease (TD) patients increased on the IgM antibody titres with the rise of their titres by Weil-Felix test, whereas the IgG antibody titres of these sera were unrelated with the titres of Weil-Felix test. We suspect that the reactivity of TD patients sera to the OXK antigen in Weil-Felix test was derived from the reactivity of the IgM antibody against the OXK antigen common with R. tsutsugamushi. The patient sera infected with a Japanese isolate of spotted fever group rickettsia (SFGR) cross-reacted with the Thai Tick Typhus (TTT) strain of SFGR by indirect immunoperoxidase test. In Weil-Felix test, the reactivity of these sera to OX2 antigen were higher than that to OX19 antigen, like the sera infected with other SFGR, except of R. rickettsii. These sera also reacted with TTT and OX2 antigens by ELISA. The titres of IgM antibody against OX2 antigen in the sera in ELISA were in parallel with the titres of the sera against OX2 antigen in Weil-Felix test, but not the titres of IgG antibody. We suggest that the reactivity of the patient sera infected with SFGR to OX2 antigen of Weil-Felix test is dependent on the IgM antibody.