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In an attempt to characterize the nature of symptomatic versus asymptomatic spotted fever group rickettsia (SFGR) infection, the immune response to R. conorii (boutonneuse fever) structural polypeptides was studied by Western-blot immunoassay. Sera from immunoperoxidase assay (IPA), SFGR seropositive (titre greater-than-or-equal-to 80) individuals, sympomatic and asymptomatic and from SFGR seronegative (IPA titre < 80) individuals living in a kibbutz community in the desert region of Southern Israel were examined by immunoblot. This community suffered from a very high morbidity rate due to SFGR (21-fold higher than the national reported average). The entire community (n = 326) has been followed-up since 1985, with serial serum samples being examined for specific IgG antibodies by IPA. The intensity of the immunoblot reaction correlated with specific IgG antibody titres as determined by IPA. This correlation was also observed between the decrease in the IgG titre and the strength of the antibody-antigen reaction by immunoblot over time for a given individual. IPA seropositive sera from asymptomatic as well as symptomatic spotted fever cases reacted to 8 individual polypeptides. In both cases antibodies to 22 kD, 24 kD, 26 kD, 28 kD, 30 kD, 32 kD, 34 kD, and 37 kD were found. In the IPA seronegative sera, antibodies to polypeptides in the range of 24 kD to 32 kD were not detected. The lack of detectable differences by immunoblotting between SFGR symptomatic vs, asymptomatic cases might be explained by other aspects of the immune response of each infected individual, and/or it is possible that virulent and non-virulent antigenically closely related SFGR strains infected symptomatic vs. asymptomatic individuals.