Biological and molecular properties of receptors specific for human T- and B-lymphoid cells are discussed. Sensitive simple and reproducible methods to enumerate B- and T-lymphocytes are described: their possible application in clinical immunology is indicated. A new method to obtain enriched populations of human T- and B-lymphocytes from peripheral blood is described. This method appears to be suitable to investigate the properties of antigens specific for B- and T-lymphoid cells.