HLA A2 (A2) transgenic mice are currently being used to compare different vaccination protocols. However, the monitoring of A2 restricted CTL in A2 transgenic mice have been hampered by poor staining efficiency of mouse CTL by A2 tetramers. We demonstrate here that chimeric A2 tetramers containing mouse H-2K(b) (K(b)) alpha3 domain (A2K(b) tetramers) can be used as staining reagents to monitor A2 restricted CTL responses in A2 transgenic mice. The increased ability of A2K(b) tetramers to stain mouse A2 restricted CTL, as compared with A2 tetramers, correlated with their higher binding affinity for mouse A2 restricted CTL. The use of these novel staining reagents will allow efficient comparison of vaccination strategies and rapid identification of novel CTL epitopes in A2 transgenic mice.