Immunoabsorption profiles were determined for Rickettsia rickettsii antigens used in four confirmatory tests for detection of antibodies to Rocky Mountain spotted fever. A human serum reactive in the four tests was absorbed with each test antigen and then reexamined by all four tests. The results indicated that the whole organism and complement-fixation antigen absorbed the whole array of antibodies to R. rickettsii, whereas erythrocyte-sensitizing substance coated on latex or erythrocytes did not. With erythrocyte-sensitizing substance coated on latex there was a decrease in titer when tested by the latex, indirect hemagglutination, and complement fixation tests but none when tested by the microimmunofluorescence test. With erythrocyte-sensitizing substance coated on sheep erythrocytes there was a decrease in titer when tested by indirect hemagglutination or complement fixation but none when tested by latex or microimmunofluorescence. These findings confirm that erythrocyte-sensitizing substance contains more than a single antigenic substance and plays a major role in the complement fixation test and a minor role in the microimmunofluorescence test.