AbstractMembrane‐bound immunoglobulins, receptors for the Fc fragment of IgG and receptors for the third component of human or murine complement were used as B cell membrane markers to study eripheral blood lymphocytes from twentytwo patients with chronic lymphatic leukemia (CLL), five patients with acute lymphoblastic leukemia (ALL) and one patient with Sézary syndrome. The capacity of human T cells of forming „spontaneous rosettes”︁ with sheep erythrocytes was employed as T cell membrane marker.In nineteen out of twenty‐seven CLL or ALL cases tested a larger percentage of cells than that found in normal individuals expressed at least one of the three B cell membrane markers studied. In the patient with Sézary syndrome the percentage of cells forming „spontaneous rosettes”︁ with sheep erythrocytes was larger than the normal, while cells bearing B cells markers were below the normal values.