Zamora, A. (University of Mexico, Mexico, D.F.), L. F. Bojalil, and Fernando Bastarrachea. Immunologically active polysaccharides from Nocardia asteroides and Nocardia brasiliensis. J. Bacteriol. 85:549–555. 1963.—Two immunologically active polysaccharides were isolated from Nocardia asteroides (Poly I Na and Poly II Na) and N. brasiliensis (Poly I Nb and Poly II Nb). These polysaccharides were isolated from cell extracts and purified by methanol precipitation, chloroform extraction of extraneous material, and deproteinization with trichloroacetic acid. The crucial step used for separation of Poly I and Poly II from both nocardias was differential solubility. From dried preparations containing both polysaccharides, Poly I was solubilized at pH 10, whereas Poly II remained insoluble and was subsequently solubilized at pH 5. Poly I Na and Poly I Nb are apparently the same. Arabinose and galactose were the monosaccharide constituents of these polysaccharides, and their molar ratios were similar. Furthermore, Poly I Na and Poly I Nb cross-reacted in agar diffusion precipitin tests with rabbit antisera prepared against either N. asteroides or N. brasiliensis. Either polysaccharide absorbed serum antibodies against the other. These polysaccharides can be regarded as groupspecific. Poly II Na and Poly II Nb are different and species-specific. They are composed of arabinose, galactose, and mannose but exhibit different molar ratios of these sugars according to species. They reacted only with homologous antisera.