The B7 adhesion molecule, a member of the immunoglobulin superfamily, has previously been identified primarily on cells of hematopoietic origin. Because B7 has been shown to facilitate interactions with T cells and because cells of the epidermis are proficient at binding and activating T lymphocytes, studies were performed to determine whether B7 was expressed in human epidermis. A subpopulation of brightly staining B7-positive cells was observed in situ in normal human epidermis. Flow-cytometric examination of epidermal cell suspensions that had been cultured for 24 h or longer demonstrated that between 10 and 40% of cells expressed B7 or a closely related antigen. Immunoelectron microscopy, double-staining procedures, and examination of epidermal suspensions depleted of Langerhans cells all confirmed that the B7-positive cells were keratinocytes. These studies identify human epidermal keratinocytes, a non-hematopoietic cell population, as a cell type capable of expressing a B7-like adhesion molecule. © 1993.