A monoclonal antibody was used to show directly positive thymic selection of the T cell repertoire in mouse strains expressing the 17a beta-chain variable domain (V beta 17a) of the T cell receptor. In the absence of the potent tolerizing class II major histocompatibility complex (MHC) molecule, I-E, peripheral expression of V beta 17a+ T cell receptors varied with the MHC haplotype of the mouse strain. In the most extreme case, H-2q mice expressed high peripheral levels of CD4+ V beta 17a+ T cells (14 to 19 percent), whereas H-2b mice expressed low levels (3 to 4 percent). Analysis of (b x q)F1 mice and chimeric mice showed that these differences were determined by positive thymic selection and implicated the thymic epithelium as the controlling cell type.