Predisposition to Type I insulin-dependent diabetes (IDD) has a strong underlying genetic basis involving class II major histocompatibility complex (MHC) genes as well as several non-MHC genetic systems. In the non-obese diabetic (NOD) mouse, a model for human IDD, genes associated with the appearance of immune cell infiltrates in the pancreatic islets (insulitis) and/or overt IDD have been mapped to chromosomes 1, 3, 6, 11, and 17. A recent report has suggested that CD8+ lymphocytes of the NOD mouse might be deficient in the expression of the CD8 beta molecule, a protein encoded by a gene on chromosome 6. The CD8 beta molecule is a T-cell surface marker, the lack of which could affect selection in the thymus, possibly permitting auto-reactive T-cell clones to populate the peripheral lymphoid tissues. For this reason, we examined the expression of the CD8 molecule by lymphocytes in the NOD mouse. Results indicate that the NOD mouse is not deficient in its transcription of detectable mRNA encoding either the CD8 alpha or beta subunits. However, the NOD mouse expresses the Lyt2 alpha and Lyt3 alpha alleles, suggesting that a portion of chromosome 6 centromeric to the diabetes-susceptibility genetic region is derived from an ancestry common to AKR and, like AKR, the CD8 alpha and CD8 beta 3.1 (but not CD8 beta 3.2) subunits are detected on the cell surface of T lymphocytes of the NOD mouse. Interestingly, though, the CD8 beta 3.1 molecule may not be expressed in the NOD mouse to the same extent as it is expressed in the AKR/J mouse, suggesting the possibility that the NOD mouse possesses a defect somewhere between transcription and cell surface expression of the CD8 beta molecule.