The cDNA for human ferrochelatase, the enzyme that is defective in the rare genetic disease erythropoietic protoporphyria (EPP), was tested for its ability to allow the expression of ferrochelatase in mammalian cells. The cDNA was ligated to the plasmid expression vectors pCD and pED6 and transfected into COS-1 and CHO-DUKX cells, respectively. In each case, ferrochelatase activity increased. The cDNA was also ligated into the retroviral vector pLXSN, and virus-packaging cells were produced. Supernatants from these cells were used to infect fibroblasts in vitro from a patient with EPP. We found that the infected cells containing the ferrochelatase cDNA had enzyme levels in the range of normal fibroblasts and that they did not accumulate protoporphyrin when grown in the presence of delta-aminolevulinic acid. We conclude that introducing the cDNA for normal ferrochelatase into fibroblasts from an EPP patient restores ferrochelatase enzyme activity to the normal range. These experiments suggest potential for genetic therapy in EPP.