We studied the expression of catalase and myeloperoxidase genes in the hydrogen peroxide-resistant variants of human myeloid leukemia HL-60 cells HP50-2 and HP100-1. Southern blot hybridization with catalase and myeloperoxidase cDNA probes indicated that the copy number of the catalase gene in HP50-2 and HP100-1 cells was two and eight times, respectively, higher than that in HL-60 cells, whereas the copy number of the myeloperoxidase gene was the same. The amplified catalase and c-myc genes in HP100-1 cells were not decreased by treatment of the cells with inhibitors of poly(ADP-Ribose) polymerase, such as nicotinamide and benzamide. RNA blot hybridization with cDNA probes indicated that the content of catalase mRNA in HP50-2 and HP100-1 cells was four and 16 times higher, respectively, than that in HL-60 cells. By contrast, the content of myeloperoxidase mRNA in HP50-2 and HP100-1 cells was only a few percent of that in HL-60 cells. Furthermore, fluorescent in situ hybridization of a catalase cDNA probe to chromosomes indicated that the catalase gene in HP100-1 was amplified in the p13 region of a derivative chromosome 11. These results indicate that the increased synthesis of catalase in these resistant cells is mainly due to increased expression of the catalase gene, and that the lack of myeloperoxidase synthesis in these cells is due to the absence of its mRNA.