THE major histocompatibility complex (MHC) of mouse and man controls the expression of several cell surface antigens¹. The classical transplantation antigens are present on most if not all adult, nucleated cells. However, molecules controlled by the immune response region of the MHC display a much more restricted tissue distribution. The immune response-associated antigens (the Ia antigens in the mouse and HLA-D antigens in man) seem to be integral parts of the plasma membrane of B and T lymphocytes, macrophages, spermatozoa, and epidermal cells². The physiological role of the Ia antigens is far from understood but their participation in various immunobiological events is well documented (for a review, see ref. 3). In addition to their role as triggers in the mixed leukocyte reaction⁴, the Ia antigens are required for collaboration between T and B lymphocytes⁵ and between macrophages and T cells⁶. Since all known functions of the Ia antigens pertain to the immune system it seemed that they should be expressed on an epidermal cell population involved in immune reactions. We therefore set out to investigate which of the three cell types of epidermis, keratinocytes, melanocytes, and Langerhans cells, expresses the Ia antigens, and from the results of fluorescent antibody staining we conclude that it is the Langerhans cells.