As inhibitors of the dextran-human 1 → 6 antidextran system on a molar basis, the series isomaltohexaose τ; isomaltopentaose τ; isomaltotetraose > isomaltotriose was found. The ratio of inhibiting power of these oligosaccharides to one another varied in the serum of different individuals, but remained quite constant in serum samples obtained from a given individual without further immunization over a 3-year period. The data are best interpreted as indicating that the antidextran combining sites produced by a single individual are not homogeneous with respect to the extent of their complementary areas but range in dimensions from sites complementary to a trisaccharide or tetrasaccharide to those complementary at least to a hexasaccharide. The significance of these findings in relation to specificity and cross reactions is discussed.