The catalase activities of HP50-2 and HP100-1 cells, which are H2O2-resistant cell lines derived from human leukemia HL-60 cells, were 3 and 18 times higher, respectively, than that of HL-60 cells. These catalase activities of the resistant cells were precipitated with anti-catalase serum. The glutathione peroxidase activity of HP50-2 cells was about twice that of HL-60 or HP100-1 cells. The superoxide dismutase activities of HP50-2 and HP100-1 cells were, respectively, about 4 and 2 times that of HL-60 cells. In addition, both the resistant cell lines were completely devoid of myeloperoxidase activity. Pulse-labeling experiments showed that the syntheses of catalase in HP50-2 and HP100-1 cells were, respectively, 2 and 4 times that in HL-60 cells, and that, unlike the parent cells, neither line synthesized myeloperoxidase. Thus the alteration of catalase, glutathione peroxidase, and superoxide dismutase activity could be linked to the resistance of H2O2 of human leukemia cells.