The possibility that gram-negative sepsis can be diagnosed by detection of lipopolysaccharide (LPS) bound on the surface of monocytes in the circulation of patients with gram-negative sepsis was investigated. Peripheral monocytes were analyzed by flow cytometer and an anti-LPS monoclonal antibody in 3 groups: children with gram-negative sepsis, children with gram-positive sepsis, and healthy children. LPS-bound monocytes were found in all patients with gram-negative sepsis but not in children with gram-positive sepsis or in healthy children. Therefore, the flow cytometry method developed for this study may be a novel method for diagnosing gram-negative sepsis.