The antitumor drug fostriecin (phosphotrienin, FST) has been reported to exert its cytostatic and cytotoxic effects via inhibition of DNA topoisomerase II. The sensitivity of human lymphocytic leukemic MOLT-4 and promyelocytic HL-60 leukemic cells to a wide range of FST concentrations was studied by analyzing the cell cycle-specific effects and changes in nuclear chromatin induced by this inhibitor. The latter was evaluated by assaying the sensitivity of DNA in situ to acid-induced denaturation cytofluorimetrically, with the use of the metachromatic fluorochrome acridine orange (AO), which differentially stains double-stranded and denatured DNA. The cytostatic effects were observed soon after addition of FST (at concentrations of 1-30 microM for MOLT-4 cultures and 1-5 microM for HL-60 cultures) as a perturbation of cell progression through S and G2 phases of the cell cycle. Cell progression through the cycle was halted at greater than 30 microM FST in MOLT-4 cultures and at greater than 5 microM in HL-60 cultures; the effect was instantaneous and affected all phases of the cycle, so that no changes in the cell cycle distribution were apparent with increasing length of exposure to the drug. Instead, at these high FST concentrations, immediate cytotoxic effects became evident, manifesting either as cell apoptosis or necrosis. Apoptosis was observed only in the case of HL-60 cells, at FST concentrations of 5-100 microM, and was characterized by markedly increased sensitivity of DNA to denaturation combined with a decrease in overall DNA stainability, either with the DNA-specific dye DAPI or with AO, indicative of the activation of endogenous nucleases. Necrotic cell death was observed at FST concentrations of 1 mM and at greater than 30 microM for HL-60 and MOLT-4 cells, respectively: in both cases the overall DNA stainability, with either DAPI or AO, was unchanged compared to the control, but their DNA was very sensitive to denaturation. Interestingly, DNA in G2 and late S phase MOLT-4 cells, which were undergoing necrotic death, was much more sensitive to denaturation than was DNA in G1 cells of this lineage. The data indicate that chromatin changes induced by DNA topoisomerase II inhibitors in cells that undergo apoptotic or necrotic death can be conveniently monitored by the assay of DNA in situ sensitivity to denaturation.