BALB/c mice were intravenously injected with lipopolysaccharide (LPS) (0.05 µg/g of body weight) 7 days after being primed with zymosan. Recombinant human lactoferrin (250 µg/g of body weight), intravenously administered 1 day before the injection of LPS, significantly lessened the severity of hepatitis, as assessed by levels of serum alanine transaminase compared to those seen when casein was administered. The transient rise of serum tumor necrosis factor alpha (TNF-) after LPS treatment was also significantly lowered by the intravenous administration of lactoferrin, suggesting that the effect of lactoferrin was due to the suppression of TNF- production. The following results indicate that the sites of action of lactoferrin for the suppression of the development of this type of hepatitis are Kupffer cells. Gadolinium chloride, a substance known to eliminate Kupffer cells, administered 1 day before LPS, inhibited the transient rise of TNF- and protected against the development of hepatitis. Kupffer cells isolated from mice intraperitoneally injected with recombinant human lactoferrin became refractory to LPS. The specific interaction of recombinant human lactoferrin with the Kupffer cells was shown by a binding assay, which revealed two types of binding sites on mouse Kupffer cells. Of the two dissociation constants determined in this way, the lower dissociation constant, 0.47 × 10⁶ M, was within the range of the 50% effective doses for the suppression of TNF- production. These results suggest that recombinant human lactoferrin administered to mice suppresses the production of TNF- by Kupffer cells by directly associating with the binding sites on these cells.