Reactivity of a monoclonal antibody with human Langerhans cells was demonstrated by a double-labeling immunofluorescence technique. Ia-bearing cells of the epidermis (Langerhans cells) were reactive with this antibody both in frozen sections and in cell suspensions prepared from human epidermis. This monoclonal antibody was unreactive with non-Ia-bearing epidermal cells and with peripheral blood B cells, T cells, and monocytes but did not bind to 70% of intrathymic lymphocytes. These observations further distinguish Langerhans cells from classical monocytes. Furthermore, this monoclonal antibody is a highly specific marker for the in vivo identification and in vitro isolation of Langerhans cells.