alpha-N-(3-Acyloxyacyl)-ornithine (or -serine) is the structure of lipoamino acids obtained by us previously from some gram-negative bacteria (Y. Kawai and I. Yano, Eur. J. Biochem. 136:531-538, 1983; Y. Kawai, I. Yano, and K. Kaneda, Eur. J. Biochem. 171:73-80, 1988; Y. Kawai, I. Yano, K. Kaneda, and E. Yabuuchi, Eur. J. Biochem. 175:633-641, 1988). The 3-acyloxyacylamide structure is present in both the lipoamino acids and lipid A of lipopolysaccharide (endotoxin). The efficacy of lipoamino acids (an ornithine-containing lipid and a serine-containing lipid) in activating C3H/HeSlc mouse peritoneal exudate macrophages was compared with that of bacterial lipopolysaccharide, because the two types of substances were expected to exhibit similar biological activities and physiological functions on the basis of their structural similarities. Actually, the lipoamino acids, as well as lipopolysaccharide, strongly activated the macrophages to generate the immunoregulatory substances prostaglandin E2 and interleukin-1, but their effect on the induction of L929 cell cytolytic factor (a possible tumor necrosis factor), another immunoregulatory substance, was weaker than that of lipopolysaccharide. The effect of lipoamino acids on the cytotoxicity of macrophages for EL-4 leukemia cells was very weak. However, all of these activities, as far as tested, were strongly enhanced by synergistic action with gamma interferon. Only the serine-containing lipid killed both C3H/HeSlc and C3H/HeJ macrophages to almost the same degree as endotoxin killed C3H/HeSlc macrophages. On the other hand, lethal toxicity for mice was not found with either the ornithine-containing lipid or the serine-containing lipid, even when 7 mg of compound was injected into a mouse. These studies suggest that the lipoamino acids are nontoxic characteristic immunoactivators.