The phenomenon of redistribution of surface membrane immunoglobulin (Ig) components (capping) has been well described in mouse lymphoid cells. The characteristics of this process in human lymphocytes are less clear. This study characterizes the phenomenon of surface membrane Ig redistribution of normal and chronic lymphocytic leukemia (CLL) lymphocytes with the use of fluoroscein-labeled anti-Ig sera. Normal lymphocytes underwent rapid cap formation after incubation with anti-Ig serum in the cold and subsequent rewarming. The morphology was characteristic with aggregation over the pole of the cell opposite the nucleus and over the uropod when present. The process was energy dependent but independent of protein synthesis, and could be inhibited by vincristine, vinblastine, and colchicine but not by cytochalasin B. CLL cells, on the other hand, though showing fluorescent complex aggregation on the surface, rarely demonstrated unidirectional movement of these aggregates to form a cap. Cap formation in these cells could not be stimulated by supplementing the energy source or protein concentration of the medium nor by adding glutamic acid which could partially reverse the vincristine and vinblastine inhibition of normal capping. The failure of agents which inhibit motility to inhibit capping of the normal lymphocytes suggests that active locomotion is not a direct prerequisite for capping. The results also suggest the involvement of microtubules in normal capping and the possibility that abnormal membrane structure or microtubular function could explain the failure of CLL cells to behave normally in this regard. The role of this cellular defect in the immune deficiencies exhibited by many patients with CLL, however, is not established.