Summary: A variety of dextrans cross-react with Types II and XX antipneumococcal sera and, to the extent of their cross-reactivity, remove the same fraction of anti-SII or anti-SXX. There appears to be no correlation between the proportions of the various linkages in the dextrans and the maximum quantity of antibody precipitable by any given dextran. Glycogen precipitates a portion of the anti-SII which cross-reacts with dextrans. The relationship between the proportions of 1,6 linkages in the dextrans and their efficiency, on a weight basis, to precipitate 50% of their maximum cross-reacting anti-SII or anti-SXX was very similar to that found in another investigation for the dextran-human antidextran reaction. In general, dextrans with 70% or more α-1,6 linkages are equally efficient in precipitating cross-reacting antibody but precipitating efficiency declines with decreasing proportions of 1,6 linkages below 70%. Several dextrans were found which did not follow this relationship and all but two of the exceptional dextrans behaved similarly both in the cross-reactions and the dextran-antidextran reaction. Quantitative inhibition studies with oligosaccharides show that oligosaccharides of the isomaltose series are the best inhibitors of the cross-reactions of anti-SII and anti-SXX with dextrans and anti-SII with glycogen. An upper limit in inhibiting power of the cross-reactions of anti-SII and anti-SXX with dextran N236 was reached with isomaltopentaose and isomaltotetraose, respectively, indicating extents of the complementary areas of the cross-reacting combining sites on these antibodies in close agreement to that found for the dextran-human antidextran system. With other dextrans, units as small as α-methylisomaltoside gave maximum inhibition of the cross-reactions. The crossreaction of anti-SII with glycogen was inhibited maximally by α-methylisomaltoside. Differences in the proportion of total anti-SII cross-reacting with dextrans were found in two horse Type II antisera, confirming previously reported heterogeneities in the antibody responses of different animals to the same antigen (SII). The cross-reaction of anti-SII with Friedländer Type B polysaccharide seems to involve a fraction of anti-SII independent, at least in part, from the fraction cross-reacting with dextrans. Quantitative inhibition studies with oligosaccharides showed that glucuronic acid residues may be involved in this cross-reaction since glucuronic acid was the only inhibitor tested which was effective.