The common class I alleles (e.g., Kd and Db) within the H2g7 major histocompatibility complex (MHC) clearly contribute to autoimmune IDDM in NOD mice, but the mechanism by which this occurs has been controversial. One laboratory has reported that the peptide transporter encoded by the Tap1 gene within H2g7 is defective, and this contributes to IDDM by impairing MHC class I-mediated antigen presentation. If true, defective MHC class I-mediated antigen presentation should segregate with the H2g7 haplotype. NOD mice, related congenic stocks, and other control strains were used to test this hypothesis. H2g7-positive strains did not differ from those expressing other MHC haplotypes in ability to present MHC class I-restricted H3aa or H3ab minor histocompatibility (H) antigens to cytotoxic T-lymphocytes (CTL). The H2g7 haplotype was found to have a reduced capacity to mediate MHC class I-restricted presentation of the H47a minor H antigen. However, MHC class I-restricted presentation of H47a was found to be Tap independent. NOD mice and control strains also did not differ in ability to activate adenovirus-specific MHC class I restricted CTL. Thus, the H2g7 haplotype is not characterized by a Tap gene defect that only impairs the inductive phase of the immune response. In addition, MHC class I-restricted presentation of either minor H or adenoviral antigens was equivalent in male and female NOD mice. Therefore, while the class I alleles of the H2g7 haplotype exert diabetogenic functions in NOD mice, this is not elicited through a Tap gene defect. The absence of female-specific Tap gene defects also indicates this cannot account for the reduced male incidence of IDDM in some NOD mouse colonies.