It has been reported that the level of Tap-1 (transporter associated with antigen processing) mRNA and the expression of class I on splenocytes are low in NOD mice. Class I expression at 37°C depends on an adequate supply of peptides, so a decrease in Tap could lead to lower class I levels. Since hypoexpression of class I correlated uniformly with the development of diabetes, it has also been suggested that Tap-1nod is diabetogenic. However, others report normal Tap-1 and class I levels in NOD mice. We examined Tap-1 and Tap-2 mRNA levels in NOD/Smrf mice using a reverse transcriptase-polymerase chain reaction method that detects ≥25% changes in mRNA. We also assessed class I expression with three monoclonal antibodies. No difference in Tap-1 or Tap-2 mRNA levels for females of different ages or between diabetic and nondiabetic animals was observed. Tap-1 mRNA levels were identical between NOD/Smrf and BALB/cJ mice. Kd expression was significantly lower on NOD lymphocytes than in BALB/cJ cells, but the difference was due to the smaller size of the NOD splenic lymphocyte. When cells of the same size were analyzed, no difference in class I levels was observed. Class I levels were also identical in diabetic and age-matched nondiabetic NOD and BALB/c females. Both NOD Tap-1 mRNA and class I were increased by interferon-γ. We find no evidence for impaired NOD Tap gene activity or class I expression, as previously reported for this strain.