Cryptococcus neoformans, an opportunistic pathogen, is the fourth leading cause of death among AIDS patients. The yeast's capsule is a major virulence factor, and serotype is related to the chemical structure of glucuronoxylomannan (GXM), its capsular polysaccharide. The GXM from Cap70, a hypocapsular mutant of serotype D isolate B-3501, was investigated by chemical analysis and 2D NMR spectroscopy. The assignment of 1H and 13C chemical shifts for the O-deacetylated polysaccharide was accomplished from the analysis of DQF-COSY, TOCSY, and gradient-enhanced HSQC spectra. The sequence and linkage positions of glycosyl residues were determined by NOESY and ROESY spectra. Two repeating polysaccharide components were identified as having the following structures in approximately equal proportions: [formula: see text] It is not known if these repeating units comprise a single or two separate polymer chains. Pentasaccharide 2 has been known to be the major GXM polymer of B-3501 and other serotype D isolates. Hexasaccharide 1 is identified for the first time although it has subsequently been identified in other C. neoformans isolates. The presence of 1 in the GXM of Cap70 is consistent with the extra xylose found relative to that in isolate B-3501. The mannose:xylose:glucuronic acid:O-acetyl molar ratio of Cap70 GXM is 3.00:1.73:0.78:1.75, while the same ratio for B-3501 and other serotype D isolates is approximately 3.00:1.00:0.80:1.75. Methylation analysis confirmed that the GXM of Cap70 contains unsubstituted, monosubstituted (2-linked), and disubstituted (2- and 4-linked) mannose in a ratio of 0.87:1.75:0.38. Dot blot immunoassay indicates that Cap70 is a serotype D isolate like its parent strain.