Two different HLA-A2 genes were found to be expressed in cells from donor Q66; one is identical to HLA-A2.1 and the other is identical to HLA-A2.2F (Gln->Arg at position 43, Val -> Leu at position 95, and Leu -> Trp at position 156). Studies with CTL generated from this donor demonstrated that a different CTL repertoire specific for HLA-A2 plus the MI 55-73 peptide is generated in an individual that expresses both HLA-A2.1 and HLA- A2.2F compared to individuals who express HLA-A2.` alone, and that the unique repertoire of T cell receptors can be selected by the presence of an HLA-A2 molecule with a single amino acid substitution at position 156. A peptide derived from alpha2 helical region of the H-2Kb molecule, peptide Kb163-714, was previously shown to specifically inhibit the stimulation of an alloreactive T-cell hybridoma. To further investigate the role of this region in the recognition of H-2Kb, the effects of peptide Kb163-174 on allospecific T cell lines and clones were studied. The results indicated that peptide Kb163-174 interferes with T-cell receptor engagement of a contact site on the H-2Kb molecule. It was concluded that amino acid residues 163-174 define a site used by many alloreactive T cells to engage the H-2Kb molecule. The TrAH10 monoclonal antibody is unique among conventional HLA-A*0301, but not those that express HLA-A*0302. Analysis of the specificity of this antibody has demonstrated that it binds to an epitope that is located between amino acids 144-160 on the alpha2 helix. Several lines of evidence indicated that the Arg at position 145 and the Glu at position 152 are critical for the formation of the epitope. A novel mouse H-2 class I gene, named "37", having a molecular size of 45 kD was found to be expressed in a variety of tissues indicating that class I molecules other than K, D and L may play a role in immune function. This mouse gene, encoded in the Tla region is not very polymorphic and is expressed by all mouse strains examined except A/J mice. Primary structural characteristics important for the pairing of the alpha and beta chains of human class II DR and DP molecules were defined.