Mice harboring a deletion of the gene encoding the transporter associated with antigen presentation-1 (TAP1) are impaired in providing major histocompatibility complex (MHC) class I molecules with peptides of cytosolic origin and lack stable MHC class I cell surface expression. They consequently have a strongly reduced number of CD8+ T cells. To examine whether selection of CD8+ T cells is dependent on TAP- dependent peptides, we partially restored MHC class I cell surface expression in TAP1-deficient mice by introduction of human beta 2- microglobulin. We show that selection of functional CD8+ T cells can be augmented in vivo in the absence of TAP1-dependent peptides.