1. The thermolability of the specific polysaccharides of Types I, II, and III pneumococcus has been shown by three independent methods: (a) diminution of the viscosity of solutions on heating; (b) decrease in the amount of antibody precipitated from homologous rabbit antisera; and (c) increased tendency (S III) to pass through a collodion membrane.

2. These effects may be explained most simply as a partial depolymerization under the influence of heat. In air, particularly in the presence of broth, oxidation also appears to be involved.

3. Improved and simpler methods of preparation based on these findings, are given for S I, S II, and S III. The resulting products precipitate more anti-S from homologous rabbit antisera than do the earlier preparations.

4. The methyl glycoside of methyl galacturonate has been isolated from the hydrolytic products of S I, and evidence of the ultimate structural unit obtained.

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