Lipid peroxidation was measured by the thio-barbituric assay for malondialdehyde (MDA). A small amount of MDA was formed when medial cells from guinea pig aorta were grown in tissue culture. The polyunsaturated fatty acids 8,11,14-eicosatrienoic acid, 5,8,11,14-eicosatetraenoic acid, and 7,10,13,16-docosatetraenoic acid generated significant amounts of MDA in a time-dependent manner when they were added to cultures of medial cells and fibroblasts. MDA or its precursor remained within the cell and did not accumulate in the media. Indomethacin enhanced MDA formation from polyunsaturated fatty acid. Alpha-Tocopherol, alpha-tocopherolquinone, and 2,6-di-tert-butyl-4-methylphenol (BHT) inhibited MDA formation when a polyunsaturated fatty acid was incubated with the pro-oxidant cumene hydroperoxide. Menadione had no effect on MDA formation in the cumene hydroperoxide system. Alpha-Tocopherol and alpha-tocopherolquinone inhibited MDA formation when they were added to cells in culture. Menadione had no effect on MDA formation in tissue culture. Anti-oxidant effects which were time-dependent showed that intracellular MDA was generated from a lipid peroxide precursor during the thiobarbituric acid assay. Relative plating efficiency was measured in medial cells and fibroblasts. Alpha-Tocopherolquinone and alpha-tocopherol enhanced the extent of cell proliferation. Alpha-Tocopherolquinone overcame the inhibitory effect of a polyunsaturated fatty acid on the extent of cell proliferation. Menadione was cytotoxic. Thus antioxidant data support the hypothesis that the extent of cell proliferation is controlled in part by lipid peroxidation.