We prepared a mouse monoclonal antibody that reacts specifically to human Langerhans cells (LC). The protein recognized by this antibody was mainly in the membranes of Birbeck granules and related structures. Using this antibody, we could identify LC in various tissues; these cells were in the skin, stratified squamous mucosal epithelia, lymph nodes, and the thymus. The antibody did not react with monocytes, tissue macrophages, lymphoid dendritic cells, follicular dendritic cells, or interdigitating cells. The antigen purified with this antibody was a heterogeneously glycosylated protein of Mr approximately 40,000 without interchain disulfide bonds. This antibody may be useful for identifying LC in various human tissues with or without abnormalities, and for studying the origin and fate of Birbeck granules of LC.