As already shown, the type XVIII substance, SXVIII, contains D-galactose, D-glucose, L-rhamnose, α-glycerophosphoric acid as a side chain, and immunologically important O-acetyl in the approximate proportions 1:3:1:1:1. SXVIIIA is made up of D-galactose, D-glucose, rhamnose, glycerophosphoric acid and N-acetylglucosamine in the approximate ratios 2:3 (or 4) : 1:1:1. It lacks O-acetyl groups and is more resistant to alkaline degradation than SXVIII. The rhamnose and aminosugar were identified chromatographically; the other components, with the help of appropriate enzymes as well. Of the sugars in SXVIIIA, most, if not all, of the D-glucose disappears, as in SXVIII, on oxidation with periodate, while the others appear to be stable. The cross-reactions between the polysaccharides and antipneumococcal XVIII and XVIIIA sera are extensive in both directions.