B cell and T cell characteristics of circulating lymphocytes were studied in 13 patients with chronic lymphocytic leukemia (C.L.L.). Immunofluorescence of surface immunoglobulins and rosette formation using sensitized sheep red blood cells were employed to detect B cells. Rosette formation with untreated sheep red blood cells and in vitro PHA stimulation and cytogerietics were used to study T cells. T h e findings generally confirm the conclusions of others that C.L.L. usually represents a proliferative disorder of B cells, probably clonal i n nature. In 11 of 13 cases the predominant cell surface immunoglobulin was IghI; in 1 case it was IgG, and i n 1 case there was only a slight elevation i n thc incidence of IgM positive cells. I n general, EAC rosette studies confirmed the observations made using imniunofluorescence, b u t in one case this correspondence was apparent only after the incidence of EAC rosettes was increased by pretreatment of the C.L.L. cells with 2-mercaptoethanol. I n another case, the incidence of EAC rosette-forming cells significantly exceeded the incidence of immunofluorescent cells, perhaps suggesting a variable expression of different surface markers on neoplastic cells in C.L.L. E rosette studies and PHA cultures indicated a decreased proportion of T cells in the peripheral blood of these patients, but suggested that the absolute number of T cells was not reduced. Further, kinetic and cytogenetic studies in PHA cultures demonstrated no qualitative abnormality in the circulating T cells in this disease, supporting the view that they represent a normal population diluted in a large pool of neoplastic B cells.