1. 9 Studies of lymphocyte transport are reported in which the 51Cr labelling technique of Hersey (1971) was employed on 8 patients with chronic lymphocytic leukaemia (CLL) and 1 with lymphosarcoma. 2. A new approach to the analysis of the data was formulated employing the principles of impulse analysis. This allowed the quantitation of cell transport between various locations making the minimum number of assumptions. It was also possible to take account of the principal defect of this label, its tendency to elute from live cells. 3. In spite of marked lymphocytosis, numbers of cells moving between those tissues whose lymphocytes exchange relatively slowly and the bloodstream was found to be normal. The blood and lymph nodes may therefore accumulate lymphocytes in response to a transport defect which hinders the progress of these cells from one pool to another. 4. The finding that CLL lymphocytes fluxes are normal in certain locations casts doubt on the hypothesis that lymphocyte proliferation is uncontrolled in this disease. An alternative possibility is that cell proliferation is regulated so as to maintain these fluxes within the normal range.