Leukemic cells were obtained from 12 patients with acute nonlymphocytic leukemia and 2 patients with acute lymphocytic leukemia. The specimens were incubated with a range of concentrations of adriamycin (Adr) for different periods of time and then analyzed by flow cytometry. For incubations of up to 5 h, only cells exposed to 5 or more micrograms of Adr ml-1 became fluorescent while cells incubated with 1 microgram ml-1 for 24 h became fluorescent. Fluorescence decreased when the cells were washed and further decreased when the cells were incubated in Adr-free media. The cells from different patients exhibited a wide range of Adr uptakes and retentions. Simultaneous analysis of right angle light scatter and fluorescence intensity demonstrated that there may be subpopulations of cells whose Adr uptake and/or retention differ from the weighted mean population values. With this method rare cells which fail to take up Adr can be recognized.