The application of the fluorescent cell membrane probes PKH2 and PKH 26 GL in the measurement of leukaemic cell growth was examined on four cell lines K562, NALM-6, ACV (a pre-B cell line) and HL-60 using flow cytometry. As the amount of probe per cell reduces at each cell division, the fluorescence can be used to measure cell proliferation. By measuring the mean fluorescence intensity of the cells at the beginning of culture and at various time points, and by combining this information with a viable cell count, it was possible to determine: (1) the number of viable cells; (2) their rate of proliferation; (3) their number of cell divisions; and (4) the maintenance of cells in a viable state over a period of time. It was demonstrated that these parameters could be reliably established using the red fluorescent probe PKH26 GL. In contrast, the green fluorescent probe PKH2 GL showed dye transfer resulting in an underestimation of the number of cell divisions and an overestimation of the maintenance of cells in a viable state. The potential advantages of the use of PKH26 GL over conventional assays for the measurement of leukaemic cell growth parameters are discussed.