The influence of oleic, linoleic (LIN), and eicosapentaenoic (EPA) acids incorporated into cellular lipids on susceptibility to O2-induced toxicity was evaluated in Chinese hamster fibroblasts (HA1) using a clonogenic cell survival assay. Fatty acid incorporation was achieved by incubating HA1 cells in 21% O2for 72 h in the presence or absence of media supplemented with 25 μM oleic acid, 25 μM LIN, or 2, 4, and 25 μM EPA. This fatty acid incorporation period increased the percentage of composition in phospholipids 2-fold for oleic acid, 6-fold for LIN, and 6− to 20-fold for EPA. Vitamin E, total glutathione, superoxide dismutase activity, glutathione transferase activity, and catalase activity were unchanged, relative to control, in the 25-μM EPA-treated group, and only total glutathione was elevated in the LIN-treated group. After the incorporation period, the cells were placed in non-fatty acid supplemented media and exposed to 95% O2, and clonogenic survival responses were evaluated at time intervals up to 100 h. Sensitization to O2toxicity in EPA-treated cells was apparent after 24 h of O2exposure, whereas LIN-treated cells were significantly (p< 0.05) sensitized to hyperoxia after 54 h of exposure, indicating that EPA was a more potent sensitizer for O2injury. Furthermore, cells supplemented with 4 and 25 μM EPA were more sensitive to O2toxicity than cells supplemented with 2 μM EPA. In contrast, cells treated with 25 μM oleic acid were significantly more resistant to O2toxicity at 51, 72, and 98 h of O2exposure. The results indicate that incorporation of polyunsaturated fatty acids (LIN or EPA) into the lipids of cultured cells enhances the cytotoxicity associated with exposure to 95% O2, whereas incorporation of a monosaturated fatty acid (oleic acid) reduced the cytotoxicity of 95% O2. These results suggest that O2-induced injury can be modified by alterations in peroxidizable substrates present in cells. (Pediatr Res 32:366–372, 1992)