Thy-1 antigen is expressed on a dendritic subpopulation of cells in murine epidermis. Numbering between 200 and 500/mm2 surface area in abdominal skin, they are distinct from the dendritic Langerhans cells (LCs) and melanocytes. Since immigrant lymphoid cells as well as constitutive cells in various organs have been demonstrated to be Thy-1+, their origin and function are not certain. To assess these issues, two experimental protocols were established. First, grafts of whole skin from AKR mice were placed orthotopically on (AKD2)F1 recipients. Immigration of recipient-derived cells into graft epidermis was assessed histologically by fluorescence microscopy employing monoclonal anti-Thy-1.2 and anti-I-Ad antibodies. Second, bone marrow chimeras were established in AKR recipients after lethal irradiation and reconstitution with cells from (AKD2)F1 donors. In the first protocol, dendritic I-Ad+ LCs of donor origin infiltrated each graft to normal densities within 2 weeks. Thy-1.2+ cells also immigrated into the same grafts, but at much slower rates. In the second protocol, bone marrow-derived Thy-1.2+ cells populated normal skin epidermis slowly over several months, with densities reaching 70/mm2. We conclude that some, if not all, Thy-1+ cells in normal murine epidermis are derived from bone marrow precursors, that their infiltration rates differ substantially from those of LCs, and that those factors which govern immigration rates into adult skin derive from the skin itself rather than from the systemic availability of their precursors. We suggest that the function of Thy-1+ epidermal cells will therefore reside among those usually ascribed to recirculating hematogenous cells, including the possibility that they may down-regulate immunizing signals that emerge from skin.