Three types of antibodies having specificity for carbohydrate moieties of glycoconjugate antigens have been isolated from the sera of rabbits immunized with vaccines of beta-D-glucosyl-bovine serum albumin, beta-D-galactosyl-bovine serum albumin, or beta-lactosyl-bovine serum albumin. The antibodies were isolated by affinity chromatography on adsorbents bearing appropriate carbohydrate ligands. The antibody types are specific for the beta-D-glucosyl, beta-D-galactosyl, or beta-lactosyl groups of the synthetic glycoconjugates. The specificities have been established by data on hapten-inhibition, agar-diffusion, periodate-oxidation, and affinity-chromatography experiments. Each antibody type is of the IgG class of immunoglobulins and is of uniform molecular size, with molecular weight of 1.5 X 10(5). Data from gel-isoelectrofocusing experiments showed that each preparation of antibodies, although specific for identical determinant groups of the same antigen, nevertheless consisted of a set of different proteins. The anti-D-glucose antibodies consisted of five proteins, the anti-D-galactose antibodies of eleven proteins, and the anti-lactose antibodies of seventeen proteins. The suggestion has been made earlier that the members of such a set of antibodies, in which each member is induced by the same determinant group of an antigen and each member combines with this group in the precipitin reaction, be called isoantibodies.