The structures for two cross-reactive capsular polysaccharides isolated from Streptococci pneumoniae types 19 and 57 are proposed based on the analytical data obtained from methylation, enzymatic degradation, and periodate oxidation experiments performed on both intact polysaccharides and oligosaccharides isolated from hydrofluoric acid hydrolysates. Both polysaccharides exhibit an identical structure of 4)-beta-2-acetamido-2-deoxymannose-(1 leads to 4)-alpha-D-glucose-(1 leads to 2)-alpha-L-rhamnose-1-phosphate. This trisaccharide is considered the only repeating unit in the type 19 polysaccharide. The type 57 polysaccharide contains this same repeating unit and side chains composed of a beta-D-2-acetamido-2-deoxyglucose-(1 leads to 3)-beta-D-galactose-1-phosphate side chain attached to C-2 of the glucose and an alpha-L-fucose-1-phosphate attached to C-3 of the rhamnose.