Monoclonal antibodies allowed recognition and identification of a series of hematopoietic cell membrane antigens expressed on plasma membranes of leukemia patients and on cells of healthy subjects. Monoclonal antibodies produced in different laboratories were clustered on the basis of their reactivity with normal and malignant hematopoietic cells into CD (cluster of differentiation) groups, with defined properties of recognized antigens (glycoprotein, phosphoprotein or glycolipid nature, molecular weight, isoelectric point, amino acid or carbohydrate sequence, etc.) and with functional characteristics of the corresponding antigen and its distribution on cells of the hematopoietic system. This "CD system" of human hematopoietic cell differentiation antigens was established in a series of international laboratory workshops (1982-1989). The present state of the CD classification system, as assessed after the IVth International Workshop on Hematopoietic Cell Differentiation Antigens (1988/89), in which more than 100 laboratories analyzed over 1,000 monoclonal antibodies, is described. An overview of the data concerning properties of monoclonal antibodies produced in the authors' laboratory (monoclonal antibodies series Bra-) is presented.