Summary: Isomaltose and isomaltotriose were oxidized to their respective aldonic acids with bromine and coupled to bovine serum albumin by the mixed anhydride reaction. All of six rabbit antisera to the isomaltonic acid antigen showed specificity for terminal α-glucosyl residues but did not cross-react with dextran while all of six rabbit antisera to the isomaltotrionic acid antigen cross-reacted extensively with dextran. These findings suggest a lower limit for the size of an antibody-combining site required for cross-reactivity with dextran as greater than a terminal α-glucopyranosyl unit and perhaps as large as a terminal α-linked isomaltosyl unit. With three of the antisera to the isomaltotrionic acid, antigens IM3, IM4, IM5 and IM6 were equally potent as inhibitors on a molar basis. In two of the other sera IM4, IM5 and IM6 were equally potent but slightly better than IM3 and in the sixth antiserum IM5 and IM6 were equal and slightly better than IM4, which in turn was better than IM3. The basis for these results is discussed.