The thymus is important in the differentiation of bone marrow-derived precursor cells into functional T cells; humoral factors^1–4, as well as physical interactions with nurse cells ⁵, dendritic cells and epithelial cells^6–8, are thought to be instrumental in this process. Thymic lymphocytes mature during their migration from the cortical to the medullary region of the thymus^9–11, when they undergo phenotypic changes that include the acquisition of T-cell antigen receptors^12–13, hormone receptors^{1–14, 15}and differentiation antigens¹⁶. Cortical T cells are thus mostly CD4⁺CD8⁺, whereas medullary T cells are either CD4⁺CD8⁻ or CD4⁻CD8⁺ (refs 6, 16 and 17). During this period T cells are subjected to two types of repertoire selection: all T cells recognizing self-MHC with low affinity¹⁸ may be preferentially amplified (positive selection), and in a second step T cells with high-affinity receptors for self-MHC determinants plus self antigens are eliminated (negative selection). We have described two monoclonal antibodies¹⁹ specific for the Vβ6 gene segment of the α/βheterodimeric T-cell antigen receptor and have shown that most CD4⁺/Vβ6⁺ T cells recognize the Mls^a antigenic determinant but not Mls^b (ref. 20; similar results have been reported for Vβ8.1 and Mls^a (ref. 21). In both situations, tolerance to Mls^a correlated in an MHC-dependent fashion with absence of Vβ6 or Vβ8.1 T-cell antigen receptor expressing T cells in the periphery. We show here by immunostaining of thymus cryosections and cytofluorometric analysis that Vβ6-expressing cortical T cells are present at high density in both Mls^a and Mls^b mice, but do not enter the medullary region of Mls^a animals.