The cell-envelope antigens of Peptostreptococcus anaerobius were extracted from intact cells by autoclave or alkaline treatment. The purified species-specific antigen (G) was identified among several polysaccharides obtained from the extracts by successive treatments with ribonuclease and pronase followed by ion-exchange and gel-filtration chromatography. G was investigated by 13C- and 31P-n.m.r. spectroscopy, titrimetry, elemental analysis, and gas-liquid chromatography. Oxidation of G with NaIO4 followed by reduction with NaBH4 and mild acid hydrolysis yielded the Smith degradation product of G (GS). Treatment of G and GS with 48% HF gave the respective dephosphorylated products GF and GSF. The structures of GS, GF, and GSF were investigated by 13C-n.m.r. spectroscopy, methylation analysis, and gas-liquid chromatography-mass spectrometry. The principal constituents of G were 2-acetamido-2-deoxy-D-glucose (D-GlcNAc), D-glyceric acid, and phosphate as a diester, in the ratio 2:1:1, and a minor amount of D-glucose (beta-D-Glcp). GS contained D-GlcNAc, D-glyceric acid, glycerol, and phosphate in a 1:1:1:1 ratio. GF and GSF contained D-GlcNAc and D-glyceric acid in the ratios 2:1 and 1:1, respectively. A structure for the principal repeating unit of polymeric G compatible with the analytical data consists of alpha-D-GlcpNAc-(1----3)-alpha-D-GlcpNAc-(1----2)-D-glyceric acid units linked through C-6'-C-6" phosphate diester bridges. This structure is novel for two reasons: (a) unsubstituted glyceric acid residues occur as aglycons in the repeating structure, and (b) phosphate diester bridges link nonanomeric glycose carbons in a non-nucleic acid polymer. The structural role of the minor amount of beta-D-Glcp in G remains unknown.