The study was designed to evaluate the implication of apoptosis in myeloid leukemic cell death induced by daunorubicin (DNR) and to identify the possible factors which may influence this process. DNR-induced apoptosis was characterized by morphology and DNA fragmentation in six leukemic myeloid cell lines which expressed different differentiation phenotypes. In phenotypically mature HL-60 and U937 cells, DNR induced typical apoptosis with characteristic morphological changes and intense internucleosomal DNA fragmentation within a narrow concentration range (0.5-2 microM). When these cells were treated with higher doses of DNR, large DNA fragments (100 kbp), but not internucleosomal fragments, were identified. DNR-induced DNA fragmentation in HL-60 and U937 was inhibited by antioxidants such as N-acetylcysteine (N-ac) or pyrrolidine-dithiocarbamate (PDTC). In the phenotypically immature KG1a, KG1, HEL and ML1 cell lines DNR induced no characteristic apoptotic morphological features as well as very low levels of internucleosomal DNA fragmentation, whereas large DNA fragments (200 kbp) were observed in KG1a treated with 7 microM DNR. Since the latter expressed P-glycoprotein (P-gp), the role of P-gp in the lack of apoptotic response to DNR was investigated. One P-gp inhibitor (verapamil) slightly improved DNR-induced DNA fragmentation in KG1a cells whereas the combination of verapamil and buthionine-sulfoximine (BSO), which depletes glutathion store, further increased internucleosomal DNA fragmentation. In conclusion, DNR induced internucleosomal DNA fragmentation in some but not all AML cells; the magnitude of this process being influenced by both intracellular drug concentration and oxidative balance.