To explore the effects of oxidative stress on metabolizing enzymes in blood cells, we examined the effect of hydrogen peroxide (H(2)O(2)) on glutathione S-transferase (GST) and cytochrome P450 (CYP) expression in a human erythroleukemic cell line, K562. After adding H(2)O(2) (up to 100 microM) to the culture medium of K562 cells, the expression levels of GST and CYP enzymes were monitored by RT-PCR. The expression of GSTP1 and CYP3A4 was induced by oxidative stress. Quantitative PCR and immunoblot analysis revealed a 3- to 4-fold increase in GSTP1 and CYP3A4 mRNA, and a 2- to 3-fold increase in GSTP1 and CYP3A4 protein levels, 3 d after the addition of 100 microM H(2)O(2). Induction was H(2)O(2) dose-dependent and also depended on the length of culture. Our results suggest that oxidative stress may affect GST and/or CYP expression in human blood cells, which may alter the metabolism of drugs and xenobiotics and thus, the toxicity of these compounds to the blood cells.