Human retinal pigment epithelium (RPE) contains two genetically distinct forms of superoxide dismutase (SOD) enzymes that scavenge harmful superoxide anions. Biochemical and immunochemical techniques were used to compare levels of copper-zinc- and manganese-containing forms of SOD (CuZn-SOD and Mn-SOD) in human adult and fetal RPE cells. It was found that Mn-SOD activity was higher in adult than fetal RPE cells, both in vivo and in vitro. Immunolocalization of Mn-SOD in cultured RPE cells showed a greater reactivity in the mitochondria of the adult cells. Primary cultures of adult RPE contained cells with various patterns of mitochondria as shown by immunolabeling for Mn-SOD. Adult RPE cells were more resistant to the effects of a superoxide generator, paraquat, which appeared to disrupt mitochondrial integrity as judged by staining with rhodamine 123. These results suggest that high levels of Mn-SOD protect mitochondria from oxidative damage that probably occurs with aging in the RPE.