We investigated the ability of different doses and durations of exposure to the chemotherapeutic drugs 1-beta-D-arabinofuranosylcytosine (Ara-C), mitoxantrone (MTN), and paclitaxel (taxol, TXL) to induce internucleosomal DNA fragmentation and apoptosis in human acute myeloid leukemia (AML) HL-60 cells in suspension culture. At clinically achievable concentrations, all three drugs have been shown to induce apoptosis in HL-60 cells. An improved method was developed for the isolation of pure genomic DNA and the detection of drug-induced internucleosomal DNA fragmentation in < 1.0 microgram of DNA sample by agarose gel electrophoresis. Morphologic evidence for apoptosis was determined by light microscopy following Wright staining, and cell viability was assessed by trypan blue dye exclusion. Internucleosomal DNA fragmentation was observed following exposure to 1.0 microM Ara-C for 4 h, which increased with 10 and 50 microM Ara-C. Incubation with 100 microM Ara-C produced internucleosomal DNA fragmentation starting at 3 h, which increased with longer periods of exposure to Ara-C. Utilizing a schedule of 1-h exposure followed by 3-h suspension in drug-free medium, 0.25 microM MTN was found to initiate DNA fragmentation, which increased with exposure to 1.0 and 5.0 microM MTN. However, identical treatment with higher concentrations of MTN resulted in random DNA degradation. Alternatively, continuous exposure to 1.0 microM MTN for 3 h was necessary to initiate internucleosomal DNA fragmentation. This increased with exposure intervals of up to 6 h. Exposure to TXL concentrations as low as 0.01 microM for 24 h caused internucleosomal DNA fragmentation, which increased with dose escalation (0.05, 0.1, 0.5, and 1.0 microM) of TXL. Although continuous exposure to 1.0 microM TXL for a period as short as 8 h produced internucleosomal DNA fragmentation, this increased significantly with longer exposure intervals. In general there appears to be a threshold concentration and duration of exposure below which none of these three drugs activates endonucleolytic internucleosomal DNA fragmentation and apoptosis. This threshold is lower for the DNA-interactive drugs MTN and Ara-C but higher for the non-DNA-interactive drug TXL. Higher doses or prolonged treatments with the drugs produce random DNA fragmentation associated with necrotic cell death. These in vitro results may further improve our understanding of the antileukemic cytotoxic effects of these drugs, which may enable a more rational design of drug regimens for optimal treatment of AML.