Francisella tularensis, a designated Category A biological agent, can cause severe infection in humans. Previous studies have demonstrated a significant immunoprotective role for B lymphocytes in animal models, but the responses of human B lymphocytes to F. tularensis components are largely unknown. The LPS of F. tularensis is atypical and has been reported to lack biological activity on myeloid cells and mouse B cells. Our study characterized the immunological effects of highly purified LPS from different stains of F. tularensis on human B lymphocytes and compared these effects with those on mouse B cells and human monocyte-derived macrophages. Results indicate that marked differences exist between cell type and species in specific responses to this interesting bacterial component. In sharp contrast to responses of mouse splenic B cells or human macrophages, human peripheral B cells showed reproducibly elevated IL-6, TNF-alpha, and antibody production in response to F. tularensis LPS. Data also indicated that these activated human B lymphocytes may subsequently promote the activation of other immune cell types by direct cell-cell interaction. Further investigation into the potential usefulness of F. tularensis LPS as an adjuvant component of a more optimal subunit vaccine is warranted, as it is now clear that it is not biologically inactive, as assumed previously.