One of the biological functions of lactoferrin is the modulation of the host defense systems, including cytokine production and immune responses. We have tested the effect of lactoferrin on the productions of tumor necrosis factor-alpha and nitric oxide in some cells. Lactoferrin itself did not induce either tumor necrosis factor-alpha production or nitric oxide production, but lipopolysaccharide-stimulated tumor necrosis factor-alpha production of macrophages and monocytes were inhibited by lactoferrin treatment combined with stimulant. The induction of nitric oxide synthesis in stimulated macrophages and vascular smooth muscle cells was not affected by the lactoferrin.