The galactose in the specific polysaccharide of type XVIII pneumococcus (S XVIII) has now been identified as the d enantiomorph by means of d-galactose oxidase. Glycerol was confirmed by paper electrophoresis and by the action of glycerol dehydrogenase. O-Acetyl was found as an additional constituent very important in the immunological specificity of S XVIII. Molar ratios of the components approximate: d-glucose 3, d-galactose 1, l-rhamnose 1, glycerol 1, O-acetyl 1, phosphate 1. The sugars appear to be in the pyranose form. Treatment with strong alkali splits off glycerophosphate and O-acetyl, yielding a product, Alk S XVIII, which retains the sugars with no change in ratios. Alk S VXIII, oxidized by periodate and reduced by borohydride, yields, in the proportions indicated: d-galactose 1, l-rhamnose 1, erythritol≮1.5, glycerol 1. Mild acid hydrolysis of this product, followed by a second oxidation with periodate and reduction, results in destruction of the galactose and rhamnose with formation of glycerol and propylene glycol. These results, together with the cross reactions described and tests for their inhibition, point toward one or the other structure: [formula omitted]. © 1963, American Chemical Society. All rights reserved.