In correlating results obtained from a new latex-Rickettsia rickettsii test with results obtained from a reference microimmunofluorescence test for Rocky Mountain spotted fever, we found that for seropositive patients each microimmunofluorescence titer (up to 4,096) was usually double the median titer obtained from the latex-R. rickettsii test. The pattern of immunoserological response indicated that latex-R. rickettsii is reactive with sera containing either anti-R. rickettsii immunoglobulin M (IgM) or IgG or both. The agglutination efficiency of the new test was greater when the anti-rickettsial IgM/IgG ratio was greater than or equal to 1. Sera from late-convalescent patients were generally minimally reactive or nonreactive in the latex-R. rickettsii test unless anti-R. rickettsii IgM was present.