Our attempts to clarify the contribution of the thymic vs. the cutaneous microenvironment in the maturation of dendritic epidermal T cell (DETC) precursors into DETC gave diverse results. In one series of experiments, we found that i.v. injection of fetal thymocytes (containing a TCR V gamma 3-expressing subpopulation), but not of adult thymocytes (containing no TCR V gamma 3+ cells) results in the appearance of CD3/TCR V gamma 3+ dendritic epidermal cells (=DETC). In other experiments, we have obtained evidence that transplantation of day 16 fetal skin onto a Thy-1-disparate recipient results in the appearance of donor-type DETC. Our further observation that the transplanted skin contains CD45+/Thy-1+/CD3- lymphocytes, but no mature T cells, therefore implies that fetal skin can provide stimuli promoting the expression of CD3/TCR genes in immature (CD3-) DETC precursors. It remains to be seen whether both or only one of these maturational pathways are (is) followed under physiological conditions.