Intraepidermal dendritic cells react with the monoclonal antibody, anti-T6, which also reacts with a majority of cortical thymocytes but not with mature T lymphocytes. Application of a four-step peroxidase antiperoxidase technique and immunoelectron microscopy to normal human epidermis demonstrates that T6 antigen is present on the cell surface of Langerhans cells but not on keratinocytes or melanocytes. These data suggest that T6 antigen is an important marker for Langerhans cells.