The studies were aimed to detect the cell cycle-associated differences in the susceptibility of HL-60 cells to apoptosis induced by diverse agents. Exponentially growing HL-60 cells were treated with the DNA topoisomerase I inhibitor camptothecin; the DNA topoisomerase II inhibitors teniposide, m-AMSA, Mitoxantrone, or Fostriecin; the presumed tyrosine kinase inhibitor genistein; a serine/threonine kinase inhibitor H7; the protein synthesis inhibitor cycloheximide; the DNA replication inhibitor hydroxyurea; the nucleoside antimetabolites 1-beta-D-arabinofuranosylcytosine and 5-azacytidine; and the alkylating agent nitrogen mustard, cisplatin, hyperthermia, and gamma irradiation. Endonucleolysis, which accompanied apoptosis induced by these agents, was assessed by two different flow cytometric methods, one based on DNA content measurements following extraction of low molecular weight DNA, and another using exogenous terminal deoxynucleotidyl transferase to label in situ DNA strand breaks. Each method allowed for both identification of apoptotic cells and analysis of the cell cycle distribution of the unaffected cell population; the method using terminal transferase also allowed for identification of the cell cycle position of apoptotic cells. Confirmed by analysis of DNA degradation by gel electrophoresis and changes in cell morphology, apoptosis was observed as early as 3 h after administration of most drugs and for some drugs was cell cycle phase specific. Cells progressing through S phase were selectively susceptible when treated with camptothecin, teniposide, m-AMSA, Mitoxantrone, H7, hydroxyurea, and 1-beta-D-arabinofuranosylcytosine. Cells in G2-M preferentially underwent apoptosis in cultures treated with H7 or with gamma-irradiation. Cells in G1 phase were preferentially affected by 5-azacytidine, nitrogen mustard, and hyperthermia. No significant cell cycle specificity was observed in the case of Fostriecin, genistein, cycloheximide, or cisplatin. The cell cycle related difference in susceptibility to apoptosis may be a reflection of both the severity of the lesion induced by a given drug and the ability of the cells to repair that lesion; both can vary depending on the cell cycle phase.