Among the differentiation antigens expressed by lymphoid cells, CD1 monoclonal antibodies (MCA) distinguish a group of antigens expressed by cortical thymocytes. Some of them (OKT6 and BL6) have been shown to react with normal human skin Langerhans cells (LC). Using indirect immunofluorescence on skin sections and LC-enriched epidermal cell suspensions obtained by skin trypsinisation, we have studied the reactivity of eight new CD1 MCA. Our immunocytochemical observations indicate that four groups of CD1 MCA can be distinguished on the basis of their reactivity on LC. NA.1.84, SK9/Leu6, 10 D.12.2 and I19 identify LC in skin and in suspensions, but 4A.76 and NUT2 were negative in both. M.241 became negative after trypsinisation of epidermal cells and D47 reacted with an antigen expressed on LC only after enzymatic treatment. These results demonstrate the heterogeneity of the antigens recognized by CD1 MCA. Some of them do not react with normal human LC. Some antigens appear to be masked in the epidermis and are expressed only after trypsin treatment. Our data confirm the heterogeneity of CD1 molecules recently documented on the basis of biochemical analysis.