207 LYMPHOCYTES IN CHRONIC LYMPHATIC LEUKÆMIA al.1 showed that a lymphocyte population Sm,—Wybran et and normal reactivity in vitro was characteristics T-cell with found in patients with chronic lymphatic leuksemia (C.L.L.). 11 patients with C.L.L. and 10 controls were examined by the method used by Wybran et al. Two distinct cellular populations were observed among the circulating lymphocytes. Analogous to that observed by Wybran et al. the first population (P) consisted mostly of T cells, and the second population (R) were composed mainly of lymphocytes with B-cell characteristics. The incorporation of 3H-thymidine in vitro was examined in populations 0 (unseparated lymphocytes), P, and R, after phytohaemagglutinin and pokeweed-mitogen stimulation. E and E.A.C. rosette formation and the presence of surface Ig were also studied. Our results agree with those of Wybran et aI.l and others.2-4 The behaviour of the same populations in one-way mixed lymphocyte culture (M.L.C.) was also investigated. Population 0 cultured with allogeneic lymphocytes gave a very poor response (stimulation index