To determine if overexpression of the human heme oxygenase (HO-1) protects retinal pigment (RPE) cells from hemoglobin toxicity, a human RPE cell line was infected by an adenoviral vector containing the HO-1 (Ad-HO-1) gene or transfected with a plasmid containing the cytomegalovirus promoter and HO-1 cDNA (pRc/CMV-HO-1) complexed to cationic liposomes. Phase contrast microscopy and acid phosphatase activity were examined to insure homogeneity of the cell line. Mitochondrial cytochrome and microsomal heme content were measured in both transduced and control cells. RPE cells were then challenged with hemoglobin and their viability estimated. We determined that cells transfected with Ad HO-1 overexpressed HO-1 compared to control cells: HO-1 mRNA levels were increased 3-fold within 3 days, decreasing in 7 days. In addition, we permanently transfected RPE cells with HO-1 gene. Transfected cell clones selected for neomycin resistance had elevated levels of HO activity 3-fold higher than control. Transfected cells exposed to hemoglobin had a survival rate of 93%; non-transfected cells had a 65-75% rate of survival. Transfected cells overexpressing HO-1 proved highly viable when challenged with hemoglobin. HO-1 appears to be an important component of the cellular anti-oxidant defense mechanisms against hemoglobin toxicity. However, the choice of transient or permanent expression of HO-1 against hemoglobin toxicity and hemorrhage needs to be further evaluated.