Approximately 15 per cent of the light chains from homogeneous immunoglobulins in patients with multiple myeloma contain an oligosaccharide group. Five human myeloma light chains that contained carbohydrate were studied. The sequence Asn-[unk]-Ser/Thr was at the site of carbohydrate attachment in all light chains. The carbohydrate group was attached to the asparagine residue of this triplet sequence which in all five light chains was located in the variable region. The occasional occurrence of carbohydrate in myeloma light chains is seen as the consequence of a variable region mutation creating an Asn-[unk]-Ser/Thr sequence to which carbohydrate is attached by an enzyme capable of recognizing the characteristic triplet sequence.