The purpose of this study was to determine whether endotoxin could augment toxic shock syndrome toxin 1 (TSST-I)-induced production of interleukin 1 (IL-I) by murine macrophages. Macrophages from C3H/HeJ or C57Bl/6 mice were stimulated with purified TSST-I alone or in combination with lipopolysaccharide (LPS). A dramatic synergistic thymocyteproliferative response was induced by supernatants from C57Bl/6 macrophages stimulated with both TSST-I and LPS. No enhanced response was induced by supernatants from C3H/HeJ macrophages. A portion of the enhanced response induced by C57Bl/6 macrophage supernatants was attributed to synergism between IL-I and residual TSST-I in the thymocyte assay. The addition of monoclonal antibody to TSST-I to the supernatants eliminated the effects of residual TSST-I in the thymocyte assay and demonstrated a synergistic induction of IL-l. These data (1) show that LPS can enhance macrophage responsiveness to TSST-I; (2) suggest that TSST-I not only induces IL-I secretion but also enhances target cell responsiveness to IL-I; and (3) further support the role of IL-I in the pathogenesis of toxic shock syndrome.