Hypothesis: Clostridium difficile toxins require interleukin 1 (IL-1) production or a functioning IL-1 receptor to elicit acute-phase protein production by murine hepatocytes. Design: Experimental study. Setting: Research laboratory at the DVA Medical Center, St Louis, Mo. Cells Studied: Hepatocytes prepared from normal mice, from knockout mice deficient in IL-1 production due to loss of IL-1 converting enzyme, or from knockout mice deficient in the IL-1 p80 receptor. Interventions: Cells were treated with lipopolysaccharide, a crude C difficile toxin extract, or purified C difficile toxins A or B for 24 hours in vitro, then radiolabeled with 35S methionine. Newly synthesized acute-phase proteins were identified by electrophoresis and autoradiography. Main Outcome Measures: Synthesis of a 23-kd acute-phase protein in response to the various stimuli. Results: Lipopolysaccharide, C difficile culture extract, and purified toxins A and B stimulated the synthesis of the 23-kd acute-phase protein by hepatocytes from normal mice and by hepatocytes from knockout mice deficient in the IL-1 converting enzyme. However, hepatocytes from knockout mice deficient in the IL-1 p80 receptor failed to produce this acute-phase protein when treated with the C difficile toxins, although they responded fully to lipopolysaccharide. Conclusions: Stimulation of acute-phase protein synthesis by C difficile toxins does not require IL-1 production, but does require a functioning IL-1 p80 receptor. This suggests that some of the actions of these toxins are mediated by this receptor.