The lipopolysaccharide of free-living Rhizobium leguminosarum was isolated and purified, and its homogeneity was determined by gel electrophoresis and, after mild acid degradation, by gel filtration. On electrophoresis, two molecular species were observed. After acid degradation and gel filtration, three components could be isolated: one was very rich in glucose, one contained 2-O-methylfucose, fucose, mannose, galactose, glucose, L-glyCer0-Dmanno-heptose, 2-keto-3-deoxyoctonic acid, L-alanine, quinovosamine and uronic acids, and the third component consisted of low molecular weight material. In the lipid A fraction, D-glucosamine, P-hydroxymyristic acid, /I-hydroxypalmitic acid and P-hydroxystearic acid were detected as major components. The phosphorus content was low. No major chemical differences were observed in the neutral sugar and fatty acid compositions of the lipopolysaccharides isolated from bacteria and bacteroids. The lipopolysaccharide of bacteroids was rapidly lost during isolation from the nodules and on dialysis, and behaved anomalously during ultracentrifugation.