Seven patients with chronic lymphocytic leukemia were investigated by isotopic and electron microscopic techniques to study dynamic and morphologic aspects of the kinetics of their lymphocytes. Autotransfusion studies of labeled leukemic lymphocytes showed rapid equilibration of blood lymphocytes between the vascular space and a "readily accessible pool." With increasing absolute blood lymphocytosis (900/cu mm-670,000/ cu mm), the relative importance of the readily accessible pool to the vascular space gradually decreases from eleven to one. Electron microscopic studies of the postcapillary venules in lymph nodes suggest a passage of lymphocytes from the vessel lumen, through the endothelial cells and into extravascular sites. About 20% of lymphocytes counted within the perivascular sheet were found in the wall, irrespective of the blood lymphocytosis and akin to the situation found in hematologically normal persons. These studies indicate that the readily accessible pool is complex and consists of at least two spaces, one of which is the classical recirculation space. The vascular pool of leukemic lymphocytes, as do the blood lymphocytes in hematologically normal persons, participates in this recirculation. There is a second space that has yet to be defined anatomically.