Maturation of some T cell populations has been suggested to occur in epithelial tissues. The CD8 molecule, which is expressed as heterodimers made of alpha/beta-chains on virtually all peripheral T cells, is preferentially expressed as homodimers made only of alpha-chains on a subset of CD8+ mouse gut intraepithelial lymphocytes, which arise from bone marrow (BM) precursors in a thymus-independent mechanism. This unique population of CD8 alpha+ beta- T cells, however, has not been identified in other lymphoid tissues in normal adult mice. Because our previous study demonstrated that reconstitution of thymectomized irradiated mice with T cell-depleted BM cells resulted in the appearance of CD8+, TCR-alpha beta+ dendritic epidermal cells (DEC) and suggested that a proportion of the BM-derived DEC may mature within the epidermis, we asked whether the unique phenotype of CD8 alpha+ beta- could be detected among the CD8+ BM-derived DEC. In this report for the first time we identified this unique population of CD8 alpha+ beta- cells among the DEC. Although this population comprised the predominant fraction of the BM-derived DEC in the early post-transfer period (2 to 3 mo), gradual shift from the CD8 alpha+ beta+ phenotypes occurred during the late post-transfer period (4 to 6 mo) independently of the presence of the thymus. Kinetic studies on the BM-derived DEC revealed that this phenotypic shift could be caused by the subsequent expansion of the CD8 alpha+ beta+ DEC. In contrast, the CD8 alpha+ beta- population was the major subset of the BM-derived intraepithelial lymphocytes throughout the entire observation period and the phenotypic shift with age was never observed. These results indicate that CD8 alpha+ beta- cells are preferentially detected on T cells that home to the epithelial tissues and that in the epidermis, but not in the gut epithelia, the subsequent expansion of the CD8 alpha+ beta+ DEC would dilute the high number of the CD8 alpha+ beta- cells.