The cell viability of human cancer cell lines treated with [5,10-bis(N-methyl-4-pyridyl)-15,20-diphenyl]porphinatoiron(III) (cis-FeMPy 2 P 2 P) has been estimated. The cis-FeMPy 2 P 2 P is a superoxide dismutase (SOD) mimic in vitro that exhibited a significant toxicity in cancer cell lines. This toxicity is rather due to pro-oxidant properties of the iron-porphyrin in vivo. We have demonstrated that there was the relationship between the LD 50 values calculated from the viability of cancer cell lines treated with cis-FeMPy 2 P 2 P and the SOD activities of the cell lines. Furthermore, the inhibition of SOD by antisense S-oligonucleotide increased the cytotoxic effect of cis-FeMPy 2 P 2 P against cancer cells. These results suggest that SOD is a target enzyme for the cell death induced by cis-FeMPy 2 P 2 P as a new class of anticancer agents.