The amino-acid sequence of human beta(2)-microglobulin, a protein of 100 residues, is related to parts of the constant region of IgG heavy chain. The dog homologue of human beta(2)-microglobulin has now been isolated and its amino-acid sequence has been partially determined. The two proteins (human and dog beta(2)-microglobulins) are demonstrably different at only 6 or 7 of the 42 positions examined. Use of the dog should permit experimental investigation of the functional relationships of beta(2)-microglobulin to immunoglobulins and the immune system. beta(2)-Microglobulin has also been detected in serum from a chimpanzee. The gene for beta(2)-microglobulin may correspond to the end portion of an immunoglobulin heavy chain gene.