Two allelic forms of mouse beta 2-microglobulin (beta 2m), the small polypeptide chain of H-2 histocompatibility antigens, have been identified. The two forms can be distinguished by NaDodSO4/polyacrylamide gel electrophoresis. All inbred mouse strains express a single beta 2m isotype. Mice heterozygous for beta 2m synthesize both forms, showing codominant expression of beta 2m alleles. In mice heterozygous for both H-2 and beta 3m, individual H-2 histocompatibility antigens associate with both beta 2m forms. Preliminary structural studies indicate differences in peptide composition between the two forms.