TAP-1 deficient (-/-) mice cannot transport MHC class I antigens onto the cell surface, which results in failure of the generation of CD8+ T cells in the thymus. In a series of recent studies, it has been proposed that extrathymic T cells are generated in the liver and at other extrathymic sites (e.g. the intestine). It was therefore investigated whether CD8+ extrathymic T cells require an interaction with MHC class I antigens for their differentiation in TAP-1(-/-) mice. Although CD8+ thymically derived T cells were confirmed to be absent in the spleen as well as in the thymus, CD8 alpha beta+ T cells were abundant in the livers and intestines of TAP-1(-/-) mice. These CD8+ T cells expanded in the liver as a function of age and were mainly confined to a NK1.1-CD3int population which is known to be truly of extrathymic origin. Hepatic lymphocytes, which contained CD8+ T cells and which were isolated from TAP-1(-/-) mice (H-2b), responded to neither mutated MHC class I antigens (bm1) nor allogeneic MHC class I antigens (H-2d) in in vitro mixed lymphocyte cultures. However, the results from repeated in vivo stimulations with alloantigens (H-2d) were interesting. Allogeneic cytotoxicity was induced in liver lymphocytes in TAP-1(-/-) mice, although the magnitude of cytotoxicity was lower than that of liver lymphocytes in immunized B6 mice. All allogeneic cytotoxicity disappeared with the elimination of CD8+ cells in TAP-1(-/-) mice. These results suggest that the generation and function of CD8+ extrathymic T cells are independent of the existence of the MHC class I antigens of the mouse but have a limited allorecognition ability.