T lymphocyte maturation activity of supernatants from cultured human epidermal cells was investigated using a biological rosette assay based upon Thy 1 induction on T cell precursors. Between days 5 and 18 of keratinocyte cultures, supernatants exhibited Thy 1-inductive properties. Optimal activity was found between days 10 and 14 of culture, which correspond to the beginning of epidermal cell stratification. This activity was independent of the presence of prostaglandin E. Absorption experiments using monoclonal and polyclonal antibodies showed that ELDIF was different from other recognized thymic hormones (thymulin, thymopoietin, thymosin alpha 1), produced by thymic epithelial cells and known to induce T cell markers.