Since the independent discoveries by Gorer and Snell of the murine major hist6compatibility complex (MHC), H2, monumental strides have been taken in defining both the structure and function of MHC genes and their products. First incorrectly regarded as a blood group locus (12), the H-2 was later observed as a locus controlling allogeneic organ grafts (14). The MHC is currently ascribed the crucial role of orchestrating cell-cell interactions of the immune response. Although the MHC encompasses class I, II, and III genes, as well as several enzyme-encoding genes (16), the class I and II genes and their products are the focus of this paper. It is in the context of these latter MHC gen e products that immunocompetent cells discriminate self from non-self. Initially examined in murine models, the MHC was found to have multiple loci, many of which are polymorphic. The murine MHC genetic map was recently condensed from its previous 5 division H-21 region (class II), omitting the subregions l-B, l-C, and l-J (18). The H-2 presently contains the class I loci of K, D, L, Qa, and Tla and the class II loci of IAs, I-AI3, I-Ea, I-EI31, and I-El32. Following the abovementioned work, the human MHC (Fig. 1) received recognition when Dausset described the first human leukocyte antigen (HLA) in the early 1950s (8). The 9th International Histocompatibility Testing Workshop convened this year and the HLA nomenclature committee is currently composing a final draft of HLA nomenclature revisions for 1984 (13). Their changes include formal recognition of three polymorphic class I loci A, B, and C, and an expansion of the HLA-D/DR region to include DR, DQ, and DP, formerly known as DR, DC (MB or DS), and SB, respectively (Table 1). All class I and some class II gene products are defined serologically using various HLA-A, B, C, and DRand DQ-specific antisera, while Dw region specificities are defined solely by mixed leukocyte culture (MLC). These MLC-defined determinants are as yet considered distinct from the DR determinants, yet their loci are probably genetically linked. A secondary MLC technique, termed primed lymphocyte typing (PLT), is used to detect the elusive specificities of the DP (SB) locus (10, 1 I, 19). These assays are now approaching worldwide standardization in histocompatibility testing laboratories. The gene products of the class I and II genes have been biochemically characterized and compared by numerous methods, such as: (a) extraction from membranes via papain and detergent In T h i s I s s u e