Despite knowledge of the toxicity of oxygen to the retina, its effects on the retinal pigment epithelium have not been considered. We examined the effect of 20%, 10% and 5% oxygen on growth and superoxide dismutase (SOD) activity of porcine retinal pigment epithelial cells (RPE).Growth of RPE cells was very significantly lower in 20% oxygen than in either 10% or 5%; optimal growth occurred at 10% oxygen, the concentration most like their environment in vivo. Inclusion of SOD and catalase in the media very significantly stimulated growth in 20% oxygen.The SOD activity of RPE cells was significantly related to ambient oxygen. In first passage (P1) cells, SOD activity was 44% lower on day 7 than on day 1 of culture in 20% oxygen (p $ 0.05). Transfer of cells growing in 20% oxygen to 5% oxygen arrested the decrease in SOD and resulted in significantly higher SOD levels. In fourth passage (P4) cells grown in 20% oxygen, SOD was 25% and 44% lower than cells in 10% and 5% oxygen, respectively.After one week, SOD levels in the P4 cells were significantly higher than in P1. A statistical model of SOD activity in RPE cells indicated significant negative correlations with both oxygen concentration and the cell number. Growth of RPE cells was significantly influenced by oxygen level, days of culture and passage number, but not SOD activity.We conclude that traditional culture conditions support generation of free radicals in tissue culture media that suppress both growth and superoxide dismutase activity.