Increased mouse red cell (M) rosetting lymphocytes were demonstrated in the peripheral blood of patients with chronic lymphatic leukaemia. The range was wide, and patients showed considerable variation not only in the numbers of M cells but also in T and B rosetting lymphocytes. Treatment reduced M rosette lymphocytes proportionately as the total white count fell, and differential removal occurred only when the patients became leucopaenic. If we assume the M rosetting cells are the abnormal ‘leukaemic’ cells, treatment does not preferentially remove these. The M rosetting capacity appeared to be related to the presence of an immunoglobulin factor previously demonstrated on the cells and in the serum of patients with CLL which enhances in vitro viability of the leukaemic cells.