Spontaneous rosettes and membrane immunofluorescence for the various classes of heavy and light immune globulin have been determined on lymphocyte subfractions obtained from the blood of 6 normal subjects and 16 patients with CLL by separation on gradients of various density. Whereas in the normal subject lymphocyte immunological features proved similar in all fractions, in CLL they presented significant differences from fraction to fraction, with the presence only in certain fractions of the lymphocyte peak with monoclonal membrane immune globulin and without peak coincidence for IgD and IgM, probably as a result of changes in the physico-chemical features of the lymphocytes occurring during maturation. All fractions presented a sizeable complement of lymphocytes not forming rosettes and without demonstrable membrane immune globulins. The correlation between the behaviour of membrane labels and lymphocyte morphology, stage, duration and course of the disease has been analysed. The variability in lymphocyte subfraction characteristics separable at different densities may explain discrepancies of findings in researches carried out in lymphocytes that separate only at gradient of density 1077.