Oxidation and scavenger receptor-mediated uptake of low density lipoprotein (LDL) in intimal macrophages are believed to be key events in the development of atherosclerosis. We report here that oxidized LDL increases DNA synthesis, expression of HLA-DR, and interleukin-2 receptors in T cells. The stimulatory effect of oxidized LDL was not due to a direct effect on T cells but required the presence of monocytes. Oxidized LDL also stimulated the release of interleukin-1 beta from monocytes. The maximal effect of oxidized LDL on T-cell activation and interleukin-1 beta release occurred at a concentration of 1 micrograms/ml. Native LDL also had the capacity to activate T cells, although only at higher concentrations. The stimulatory effect of both native and oxidized LDL was inhibited by superoxide dismutase. Monocytes as well as T cells were found to have the ability to oxidize LDL, suggesting that the stimulatory effect of native LDL may arise as a result of LDL oxidation during incubation with monocytes and T cells. The results suggest that oxidized LDL may activate T cells in atherosclerotic lesions.