Quercetin, a widely distributed bioflavonoid, inhibits the growth of tumor cells. The present study was designed to investigate the possible involvement of apoptosis and heat shock protein in the antitumor activity of quercetin. Treatment with quercetin of K562, Molt-4, Raji, and MCAS tumor cell lines resulted in morphological changes, including propidium iodide-stained condensed nuclei (intact or fragmented), condensation of nuclear chromatin, and nuclear fragmentation. Agarose gel electrophoresis of quercetin-treated tumor cells demonstrated a typical ladder-like pattern of DNA fragments. In addition, the hypodiploid DNA peak of propidium iodide-stained nuclei was revealed by flow cytometry. Quercetin induced apoptosis in cells at G1 and S in a dose- and time-dependent manner. The apoptosis-inducing activity of quercetin was enhanced by cycloheximide and actinomycin D. A nuclease inhibitor, aurintricarboxylic acid, inhibited quercetin-induced apoptosis, whereas deprivation of intracellular calcium by EGTA had no effect. 12-O-Tetradecanoylphorbol-13-acetate and H-7 did not affect the induction of apoptosis by quercetin. The synthesis of HSP70 was inhibited by quercetin when determined by immunocytochemistry, Western blot analysis, and Northern blot analysis. Quercetin-treated tumor cells were not induced to show aggregation of HSP70 in the nuclei and nucleolus in response to heat shock, resulting in apoptosis. By contrast, when tumor cells were first exposed to heat shock, no apoptosis was induced by quercetin. In addition, pretreatment of tumor cells with HSP70 antisense oligomer that specifically inhibited the synthesis of HSP70 enhanced the subsequent induction of apoptosis by quercetin. These results suggest that quercetin displays antitumor activity by triggering apoptosis and that HSP70 may affect quercetin-induced apoptosis.