Tiazofurin, an anticancer drug which inhibits IMP dehydrogenase, decreases cellular GTP concentration, induces differentiation and down-regulates ras and myc oncogene expression, caused apoptosis of K562 cells in a time- and dose-dependent fashion. Apoptotic cells were detected by (1) flow cytometry, (2) electron microscopy, and (3) fluorescence in situ nick translation and confocal microscopy, while the DNA ladder was not detectable. The induced apoptosis was abrogated by guanosine which replenishes GTP pools through the guanosine salvage pathways, while it was enhanced by hypoxanthine, a competitive inhibitor of GPRT. The tiazofurin-mediated apoptosis may therefore be linked with the decrease of GTP and the consequent impairment of specific signal transduction pathways. Tiazofurin induced apoptosis also in lymphoblastic MOLT-4 cells, suggesting that this action is not confined to cells of the myeloid lineage, where the differentiating effects of the drug are more pronounced.