Evidence on the role of carbohydrates in the structure and function of macroimmunoglobulin (IgM) was obtained. The principal functions are the increase of hydrophilicity and preservation of a definite conformation of the protein molecule which, in its turn, is necessary for the implementation of its biological function. The protection of tryptophan residues at the molecule surface against the contacts with the surrounding water is conditioned by the above properties. The functions of each of the five carbohydrate groups in the heavy chains of IgM are reviewed.