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Ehrlichia chaffeensis, an obligately intracellular bacterium with tropism for monocytes, is the etiologic agent of human monocytic ehrlichiosis. To determine the nature and ultrastructural location of E. chaffeensis antigens, monoclonal antibodies (MAbs) to E. chaffeensis were developed. The MAbs were used for immunofluorescence and Western immunoblotting analysis of the antigens of density gradient-purified ehrlichiae. Monoclonal antibody 6A1 recognized an epitope of a 30-kD protein. This antibody reacted with a strain-specific epitope of E. chaffeensis, Arkansas strain, and did not cross-react with any other ehrlichia tested. Monoclonal antibodies 3C7 and 7C1-B recognized Arkansas strain proteins of 30 and 29 kD and reacted with E. chaffeensis (strain 91HE17) proteins of 31 and 29 kD and an E. canis protein of 30 kD. Lack of reactivity of these two MAbs with E. sennetsu and E. risticii suggests that the epitope is group-specific. Monoclonal antibody 5D11 recognized a 58-kD protein of both strains of E. chaffeensis as well as E. canis, apparently a group-specific, conformation-independent epitope. Monoclonal antibody 7C1-C reacted with 58- and 88-kD proteins of both the Arkansas and 91HE17 strains. Trypsin treatment destroyed the reactivity of E. chaffeensis antigens with all the MAbs when tested by Western immunoblotting, indicating that these antigens are proteins with trypsin-sensitive epitopes. Immunoelectron microscopy of negatively stained intact E. chaffeensis organisms showed that the 30- and 29-kD proteins are present on the surface of the ehrlichial cell wall along with the previously localized 28-kD protein.