A set of 6 monoclonal antibodies is characterized reacting only with human T-cells but not with B lymphocytes. The antibody BL-T1 binds to the E receptor, being there on all mature T cells and on the majority of thymocytes. All respectively the majority of peripheral T cells but only subpopulations of thymocytes are recognized by the monoclonals BL-T2, BL-T3, BL-T4 and BL-T5. For identification and quantification of mature human T cells especially the antibody BL-T2 is suitable. The antigen recognized by BL-E1 is missing on peripheral B lymphocytes but expressed not only on T cells but also on erythrocytes, monocytes and a fraction of granulocytes. The monoclonals discussed are compared with the monoclonals for identification of leucocyte differentiation antigens (cluster of differentiation) characterized on the 1st Int. Workshop on Human Leucocyte Differentiation Antigens (Paris, 1982).