The kinetics of turnover of a surface receptor for IgM were studied in six cases of hairy cell leukemia and four cases of chronic lymphocytic leukemia and the results were used as an indicator of membrane turnover in the pathologic cells of these two chronic leukemias. Rosette formation, shedding, and reexpression with EA (IgM) by hairy cells, a process which was shown to require protein synthesis, followed a cyclical pattern with a modal peak-to-peak reexpression time of 6 hr. Membrane turnover time in the cases of chronic lymphocytic leukemia was 12 hr or more. This work indicates that a reduced membrane turnover rate is not an essential feature of chronic leukemias. The more general application of this type of approach to the study of membrane turnover in normal cells and their pathologic counterparts are discussed.