Monoclonal antibodies (MAb) 3F11 and 06B4 recognize epitopes that are conserved on gonococcal lipooligosaccharides (LOS), present on some meningococcal LOS, and conserved on human erythrocytes. LOS of some group B and C prototype meningococcal LOS strains (LOS serotypes Ll to L8) treated with neuraminidase showed increased expression of the 3F11 and 06B4 MAb-defined epitopes. Neuraminidasetreated LOS separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver stained showed a shift in migration from a component with a mass of approximately 4.8 kDa to a component with a mass of between 4.5 and 4.6 kDa. The same strains grown in medium with excess CMP-N-acetylneuraminic acid had LOS that shifted in migration to a slightly higher component (mass, approximately 4.8 kDa). Chemical analysis of the neuraminidase-digested products from one LOS indicated it contained approximately 1.5% sialic acid. Covalent linkage between sialic acid and the LOS was confirmed by analysis of de-O-acylated and dephosphorylated LOS by liquid secondary ion mass spectrometry. These studies show that some meningococci contain sialic acid in their LOS, that the sialic acid is cleaved and lost in conventional acetic acid hydrolysis, and that the sialic acid alters the expression of MAb-defined epitopes. Recent studies have shown that when gonococci are grown in the presence of CMP-N-acetylneuraminic acid (CMP-NANA), sialic acid is incorporated into the LOS component of 4.5 kDa that binds MAbs 3F11 and 06B4 (36, 44). The sialylated component no longer binds the MAbs (4, 36). Treatment of the LOS with neuraminidase restores these epitopes, both on organisms grown in vitro (36) and on organisms in vivo in urethral exudates (4). Although the 3F11 and 06B4 epitopes are of similar specificity and are both expressed on gonococcal LOS (37), the 06B4 epitope is expressed much better than is the 3F11 epitope on meningococcal LOS of group B and C strains (30, 35 Bacterial strains. The prototype group B and C meningococcal LOS strains (38, 62) have been described previously. The surface glycolipids (lipooligosaccharides [LOS]) of