Cell wall lipopolysaccharides (LPS) prepared from oxidase-positiveMoraxella duplexand from oxidase-negativeMicrococcus calco-aceticus(a presumptive moraxella) containedD-glucose,D-galactose, glucosamine, galactosamine, lipid A, ethanolamine, fatty acids, phosphate, and protein. The lipid A moieties prepared from the LPS fractions were composed primarily of hexosamines, ethanolamine, fatty acids, and phosphate with minor amounts of the other LPS constituents. The LPS fromM.duplexandM.calco-aceticushad the same neutral sugar composition but differed markedly in their hexosamine composition. Galactosamine was the major hexosamine component ofM.duplex; also present was an unidentified amino sugar and a small amount of mannosamine. In contrast, glucosamine was the major hexosamine component ofM.calco-aceticuswith a lesser amount of galactosamine and no mannosamine. The presence of galactosamine as the major component of the lipid A ofM.duplexsuggests that this fraction has a novel structure which differs from the poly-D-glucosamine 'backbone' structure assigned to lipid A. The fatty acid compositions of the lipid A from the two species were mutually similar and consisted mainly of hydroxylauric, hydroxymyristic, and C17-cyclopropane fatty acids. The LPS fractions of the two organisms studied resemble that ofNeisseria catarrhalisand differ from those of the true neisserias.