The chemical and immunochemical properties of lipopolysaccharides (LPS) isolated from pyocin 103-sensitive and -resistant Neisseria gonorrheae were investigated. Marked differences were found in immunochemical behavior of LPS from pyocin-sensitive gonococcal strain JW31 and its isogenic pyocin-resistant variant JW31R. JW31 LPS readily precipitated wheat-germ agglutinin, soybean lectin, and rabbit anti-Streptococcus faecalis or horse anti-type 14 pneumococcal antibody. In contrast, JW31R LPS precipitated only soybean lectin. The combining-site specificity of anti-S. faecalis cross-precipitated by JW31 LPS, or type 14 pneumococcal capsular polysaccharide, was examined by hapten inhibition, and lactose found to be the most potent inhibitor. Horse anti-pneumococcal type 14 antibodies, cross-precipitated by JW31 LPS and streptococcal lactose polymer, exhibited heterogeneity with respect to combining site specificity. Gel filtration of LPS-derived core oligosaccharide showed both strain JW31 and JW31 R to possess R-type lipopolysaccharide with cores having a Mr approximately 1800. JW31R LPS contains more galactose but less hexosamine than JW31 LPS. Both JW31 and JW31R core oligosaccharides possess D-glucosamine and D-galactosamine, probably N-acetylated, as the only nonreducing end-groups, and (1 leads to 4)-linked D-glucose residues. Chemical data support immunochemical findings which indicate that lactose units occur as a structural feature of JW31 gonococcal LPS.