Three major subpopulations of lymphocytes have been identified in human peripheral blood: thymus-derived (T) lymphocytes, bursa-equivalent (B) lymphocytes, and null (neither T nor B) lymphocytes. T-lymphocytes are commonly identified by a surface receptor for sheep erythrocytes, and by heterologous antisera raised against human thymus cells and absorbed with human B-cell leukemias or lymphoblastoid cell lines. B-lymphocytes are routinely enumerated by immunofluorescent staining technics for surface immunoglobulin, or by assays for the receptor for C3. Null cells bear a receptor for the Fc portion of immunoglobulin, and are killer (K) cells in antibody-dependent cellular cytotoxicity. Accurate quantitation of T- and B-cells in disease states has been hampered by marked contamination of monocytes in populations of peripheral blood lymphocytes purified by density gradients, binding of autologous immunoglobulin to the surface of non-B cells, and disease-associated alterations of T- and B-cell surface markers. Assays for peripheral blood T- and B-cells may be of significant clinical value in the early diagnosis of lymphocytosis of unknown origin, congenital immunodeficiency disorders, organ transplant rejection crises, and Hodgkin's disease.