To generate the transgene, the 11 carboxy-terminal amino acids, including the peroxisomal localization signal, were deleted from the human catalase gene. The initiating methionine was also deleted and the first 25 amino acids of the human ornithine transcarbamylase leader sequence were added to the amino terminus to target this protein product to mitochondria. The catalase cDNA construct was then cloned into a plasmid under the influence of the chicken-actin promoter/CMV enhancer (CAG). The catalase activity in the cardiac mitochondrial fraction of transgenic animals is 50 times higher than that in their wild-type littermates. This record represents pooled data for multiple lines.