To clarify the mechanisms by which hydroquinone (HQ; 1,4-benzenediol) produces apoptosis, HQ-induced cytotoxicity, intemucleosomal DNA fragmentation, activation of superoxide dismutase (SOD), expression of Mn and Cu/ZnSOD mRNA and activation of caspase-3, -8 and -9 were investigated in the human promyelocytic leukemic cell line HL-60. Electrophoresis and activity staining of the SOD-enriched fraction showed that HQ reduced MnSOD activation more than Cu/ZnSOD activation, suggesting that it induces mitochondrial dysfunction at an early stage of apoptosis. Furthermore, the expression of MnSOD mRNA was suppressed to a greater extent than that of Cu/ZnSOD mRNA, implying that HQ causes apoptosis by inhibiting MnSOD induction. Release of cytochrome c and activation of procaspase-3 and -9, but not of procaspase-8, occurred more rapidly (as early as 6 h) in HQ-treated cells, suggesting that HQ activates the intrinsic pathway of apoptosis. Addition of the antioxidant N-acetyl-L-cysteine (NAC) significantly reduced the cytotoxicity of HQ. At a concentration that was cytotoxic to 50% of the cells (approximately 0.05 mM), HQ activated caspase-3; this effect was reduced in the presence of NAC. Interestingly, higher concentrations of HQ (0.1-0.2 mM) caused direct cell death; however, when combined with 5 mM NAC, the activation of caspase-3 was strongly enhanced, suggesting the promotion of apoptosis. The activation of caspase-3 by HQ/NAC combinations suggests that NAC, a precursor of intracellular glutathione synthesis, acts as a co-catalyst during HQ-induced apoptosis.