Our previous finding that the cerebral proteolipid could inactivate the pyrogenicity of lipopolysaccharide (LPS) in vitro was also studied by Sephadex LH-20 column chromatography and the following results were obtained. When rabbit cerebral proteolipid was chromatographed, two main protein peaks were obtained. One appeared in the chloroform (C)/methanol (M) 6:1 and the other C/M 4:1 effluent, designated as fraction IV and fraction V, respectively. When the incubation mixture of proteolipid and LPS was chromatographed, a new protein peak appeared in the C effluent. The new protein peak was suggested to be a complex of proteolipid protein and LPS, because pyrogenicity could be detected in the protein fractions only after treatment with 2% SDS. Fraction V but not fraction IV inactivated the pyrogenicity of LPS in vitro. By re-chromatography of the incubation mixture of fraction V and LPS, a complex of protein and LPS was also eluted in the C effluent. On the other hand, by rechromatography of the incubation mixture of fraction IV and LPS, such a complex was not detected in the C effluent. The present results suggest that the proteolipid apoprotein eluted in the C/M 4:1 effluent on a Sephadex LH-20 column plays an important role in the inactivation of the pyrogenicity of LPS.