Electron micrographs of phenol-water-extracted lipopolysaccharide (LPS) of Rhodopseudomonas capsulata show filamentous and netlike aggregates. Treatment of the LPS with sodium deoxycholate resulted in a reversible splitting into subunits. The LPS represents a cell wall constituent with O-antigenic specificity. In passive hemagglutination tests, high titers were obtained when erythrocytes sensitized with untreated or heat-treated LPS were incubated with antisera obtained by immunization of rabbits with whole cells of R. capsulata. The alkali-treated LPS was not active in this test. Mouse lethality tests have shown that the LPS of R. capsulata is less toxic than LPS of Escherichia coli. Also, the X-ray protection efficacy and the phagocytic activity stimulation of LPS from R. capsulata in mice are small, as compared with LPS of E. coli. Incubation of living bacteria in saline (37 C) resulted in a solubilization of an LPS-protein-lipid complex from the outer layer of the cell wall. The isolated complex contained the components which were found in the LPS. In addition, 20% amino acids and a large amount of palmitic and stearic acids, which are typical phospholipid components, were present.