Cytosine arabinoside (1-beta-d-arabinofuranosylcytosine; Ara-C) is the most important antimetabolite chemotherapeutic drug used for acute leukemia. We examined the difference in susceptibility to Ara-C-induced cell death among a number of typical human leukemia cell lines, NALM-6, MOLT-4, Jurkat, U937 and HL-60. NALM-6, which had a high expression level of p53, a tumor suppressor gene, was most susceptible to Ara-C. U937 and HL-60, with p53-null human leukemia cell lines were little affected by Ara-C. There was not always a correlation between susceptibility and the uptake of Ara-C. The production of reactive oxygen species (ROS) was increased in all leukemia cells. Pifithrin-alpha, a chemical inhibitor of wild-type p53, ameliorated the cytotoxicity of Ara-C in NALM-6 and MOLT-4, but not Jurkat, U937 or HL-60. Our data suggest that the mechanism of Ara-C-induced cell death is a common one, involving an increase in the production of ROS and p53-dependent cell death.