We have examined the usage of the V beta 5.1 region in peripheral T cell populations of several mouse strains by measuring V beta 5.1 transcript levels. The results show that V beta 5.1 is expressed predominantly by CD8+ T cells in mice of several MHC haplotypes. This finding suggests that V beta 5.1 may confer restriction to class 1 molecules present in these strains. Interestingly, however, T cells expressing V beta 5.1, like those expressing V beta 17a and V beta 11, are clonally deleted from both CD4 and CD8 subsets in mice that express MHC IE molecules. The latter result implies that V beta 5.1 confers reactivity to a class 2 molecule (IE). These results are discussed in terms of positive selection and clonal deletion. Finally we provide evidence that genes in the K or A regions of the MHC also control V beta 5.1 usage.