Two new lymphocyte antigens, provisionally designated Qat-4 and Qat-5 have been identified with two different hybridoma-derived, monoclonal AKR antiC57BL/6 antibodies. These antigens are governed by genes located to the right (distal) end of the H-2 complex, within the Qa-2,3 region. Qat-4 and Qat-5 antigens which do not seem to be identical with Qa-2,3 or TL antigens are absent from Ig/ lymphocytes and thymocytes. They are only present on a fraction of peripheral T cells. Thus, Qat-4 is expressed on 70%, and Qat-5 on 30% of splenic and lymph node T cells, Qat-4 is also found on the majority of Ig- cells from athymic nude mice. These findings illustrate the complexity of the chromosome segment between the H-2D and Tla loci and they emphasize the role of major histocompatibility complex-associated genes for the differentiation of T cells into different subpopulations with possibly distinct immunologic functions.