In addition to its hematopoietic activities, interleukin-3 (IL-3) can modulate macrophage functions. We have studied the production of interleukin-1 (IL-1), interleukin-6 (IL-6) and tumor necrosis factor (TNF) by mouse peritoneal macrophages triggered by lipopolysaccharide (LPS) in the presence or absence of IL-3. Interleukin-3 at the concentration used (i.e., 100 U/ml) did not induce the production of any cytokines, whereas it enhanced significantly the secretion of IL-1, IL-6 and TNF by LPS-stimulated macrophages. The synergistic activity of IL-3 was observed over a wide range of Escherichia coli or Salmonella enteritidis LPS concentrations. No additive effect was noticed between IL-3 and granulocyte/macrophage colony-stimulating factor (GM-CSF), another factor able to enhance LPS-induced IL-1 production. Thus, IL-3 can potentiate the inflammatory response induced by endotoxin from Gram-negative bacteria through a potentiation of cytokine production.