Quantitative data are given on the cross reactions in Type II antipneumococcal horse sera of plant gums and hemicelluloses containing multiple terminal groupings of glucuronic acid and/or 4-O-methylglucuronic acid. Great variability is shown both in the reactivities of the polysaccharides and in the antibodies in the sera of different animals immunized with the same antigen. The 4-O-methyl substituent on the glucuronic acid residues in a gum often appears to diminish cross-precipitation with antibodies to S II.