Ehrlichia chaffeensis is a recently isolated minute gram-negative obligatory intracellular bacterium of monocytes/macrophages and is the etiologic agent of human monocytic ehrlichiosis. It is not known how macrophages respond when they encounter ehrlichiae in terms of cytokine production. In this study, we examined cytokine mRNA expression by incubating E. chaffeensis with THP-1 cells and performing competitive reverse transcription-PCR (RT-PCR). At 2 h postinfection, the levels of interleukin-1beta (IL-1beta), IL-8, and IL-10 mRNAs were significant but lower than those following Escherichia coli lipopolysaccharide (LPS) stimulation. Unlike the situation with E. coli LPS stimulation, however, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), and tumor necrosis factor alpha (TNF-alpha) mRNAs were not induced. Time course and dose-response studies confirmed the absence of IL-6, GM-CSF, and TNF-alpha mRNA induction with E. chaffeensis. Viable E. chaffeensis organisms were not required for IL-1beta IO, IL-8, and IL-10 mRNA induction, since heat-killed E. chaffeensis induced identical time course responses. IL-1beta, IL-8, and IL-10 mRNAs were detected for up to 21, 21, and 24 h postexposure with E. chaffeensis, respectively, which were shut off more rapidly than with LPS stimulation. Although heat treatment of E. chaffeensis had no effect, periodate treatment completely abolished the ability of E. chaffeensis to induce IL-1beta, IL-8, and IL-10 mRNAs. The capture enzyme-linked immunosorbent assay result corresponded with the RT-PCR results, showing that viable and heat-killed E. chaffeensis produced and secreted the same levels of IL-1beta and IL-8. IL-10 production was significantly reduced by heat treatment. Periodate-treated ehrlichiae did not induce production of any of the cytokines tested. Anti-CD14 monoclonal antibody and polymyxin B did not inhibit IL-1beta mRNA expression upon exposure to E. chaffeensis. The absence of TNF-alpha, IL-6, and GM-CSF mRNA induction may delay the development of a protective immune response, thereby allowing E. chaffeensis to set up residence in macrophages.