Centrifugal elutriation was used to separate human acute leukemia cells into proliferative and quiescent subpopulations. Ten bone marrow specimens and 5 peripheral blood specimens were subjected to centrifugal elutriations. From each patient, leukemic cell subpopulations were obtained for which the [3H]thymidine labeling index differed by 10- to 30-fold. In 6 of marrow and 2 of peripheral blood specimens, cell subpopulations were obtained for which the labeling index exceeded 20%. In 5 marrow specimens, subpopulations were obtained for which the labeling index exceeded 40%. Preliminary studies of the uptake of 1-.beta.-D-arabinofuranosylcytosine and 5-azacytidine failed to show any correlation between drug uptake and the proliferative characteristics of the leukemic subpopulations.