The complete amino acid sequence of the cyanogen bromide (CNBr) fragment Ia (CN-Ia) from the murine histocompatibility antigen H-2Kb has been obtained by using radiosequence methodology. This glycopeptide is the largest CNBr cleavage product of the H-2K molecule and extends from position 139 to position 228. The sequence determined for CN-Ia was Ala-Ala-Leu-Ile-Thr-Lys-His-Lys-Trp-Glu-Gln-Ala-Gly-Glu-Ala-Glu-Arg-Leu-Arg-Ala -Tyr-Leu-Glu-Gly-Thr-Cys-Val-Glu-Trp-Leu-Arg-Arg-Tyr-Leu-Lys-Ans-Gly-(Asn)-Ala- Thr-Leu-Leu-Arg-Thr-Asp-Ser-Pro-Lys-Ala-His-Val-Thr-His-His-Ser-Arg-Pro-Asp-Asp -Lys-Val-Thr-Leu-Arg-Cys-Trp-Ala-Leu-Gly-Phe-Tyr-Pro-Ala-Asp-Ile-Thr-Leu-Thr-Tr p-Gln-Leu-Asn-Gly-Glu-Glu-Leu-Ile-Gln-Aps-Met. The data were obtained by analysis of fragments derived by thrombic, tryptic, chymotryptic, and V8 protease digestion of CN-Ia. A carbohydrate moiety is attached to Asn at position 176. Homology between this 90 amino acid stretch of H-2Kb and HLA-B7 [Orr, H. T., Lopez de Castro, J. A., Lancer, D., & Strominger, J. L. (1979) Biochemistry 18, 5711] is 68%, and differences are noted at positions 176, 177, and 178 which in the H-2 molecule are the attachment region for a second carbohydrate moiety. No carbohydrate was detected in this position for HLA-B7 [Orr, H. T. Lopez de Castro, J. A. Lancet, D., & Strominger, J. L. (1979) Biochemistry 18, 5711].