A recombinant (r) NH2-terminal fragment of bactericidal/permeability-increasing protein, rBPI23, was shown to inhibit murine macrophage nitric oxide (NO) production elicited by lipopolysaccharide (LPS) plus interferon-gamma (IFN-gamma). Normal mouse plasma amplified NO synthesis (measured as NO2- release) at LPS concentrations of 1-10 ng/mL, and antibody to the plasma LPS-binding protein (LBP) partially inhibited NO2- release in the presence of normal mouse plasma. rBPI23 (1 microgram/mL) effectively inhibited LPS-dependent NO2- release in the presence or absence of normal mouse plasma. Fifty percent inhibition of IFN-gamma/LPS-elicited NO2- production or of binding of fluoresceinated LPS was obtained with approximately 0.2 microgram/mL rBPI23. These results provide a basis for studies of rBPI23 effects on NO synthase activity in murine models of gram-negative sepsis.