The I region of the mouse H-2 complex is divided into subregions, I-A, I-B, I-J, I-E, and I-C, that are marked by the Ia-1, Ia-2, Ia-4, Ia-5, and Ia-3 loci, respectively (1, 2). The Ia-1, Ia-5, and Ia-3 locus structural products are Ia antigens, a group of polymorphic, cell-surface glycoproteins that consist of two noncovalently associated polypeptide chains of mol wt =30,000-35,000 (a-subunit)...