An ATP assay using bioluminescence was evaluated for its ability in determining the cytotoxic effect of antileukemic drugs. Leukemic cells from 32 patients with ANLL were used to compare the ATP assay with the differential staining cytotoxicity assay (DiSC). Cells were incubated with doxorubicin, daunorubicin, idarubicin, mitoxantrone and ara-C in conditions that were adapted to mimic the in vivo exposure of leukemic cells to cytostatic drugs. After incubation the cells were cultured in liquid medium for 4 days and then analyzed for ATP content using the firefly luciferase method in an automated procedure. The cytotoxic effect as measured by both methods correlated satisfactorily (r = 0.8). We conclude that the automated ATP assay can replace the more time consuming DiSC assay for in vitro drug testing in ANLL.