T cells that express the T cell receptor V_β5.2 domain react with the class II major histocompatibility complex (MHC) molecule I-E, and V_β5.2⁺T cells are deleted in mouse strains that express I-E glycoproteins. By examination of genetically defined recombinant inbred (RI) mouse strains, it was found that the deletion was dependent on the expression of I-E and one of a limited number of non-MHC gene products (cotolerogens). The gene encoding one of these cotolerogens maps to chromosome 12 and is linked to the endogenous provirus Mtv-9. These observations suggest that the I-E-mediated and minor lymphocyte-stimulating antigen (Mls)-mediated deletions of αβ T cells from the repertoire are similar; both require the expression of a class II MHC glycoprotein and a second non-MHC gene product.