Experimental and epidemiological evidence supports the hypothesis that oxidation of low density lipoprotein (LDL) appears to be important in mediating the atherogenicity of LDL. To test this hypothesis in humans, it will be necessary to perform intervention studies in large populations. We performed two studies to assess the effectiveness of supplementation with β-carotene and vitamin E, used alone in combination with each other, and with vitamin C, to protect LDL from oxidation. In phase 1, after a placebo period, eight subjects were given β-carotene (60 mg/day) for 3 months, then β-carotene plus vitamin E (1,600 mg/day) for another 3 months, and then β-carotene plus vitamin plus vitamin C (2 g/day) for 3 months. During phase 2, β-carotene and vitamin C were discontinued, subjects took only vitamin E for 5 months. During each period, LDL samples were isolated, and measurements of susceptibility to oxidation were performed. β-Carotene levels in LDL increased nearly 20-fold, but LDL susceptibility to oxidation did not change. Addition of vitamin E increased LDL vitamin levels nearly 2.5-fold, and this decreased LDL oxidation 30-40%. During the vitamin C supplementation period, plasma levels of β-carotene and vitamin E rose, but only β-carotene increased in LDL. However, the susceptibility of LDL to oxidation in this period was not decreased further. During phase 2, when subjects took only vitamin E, LDL susceptibility to oxidation was decreased by 50% as measured by thiobarbituric acid-reactive substances, conjugated dienes, and lipid peroxide formation as well by macrophage degradation. Thus, long-term supplementation with large doses of vitamin E alone, but not $-carotene, conferred increased protection to LDL in in vitro assays of oxidation. These data should be useful in planning therapeutic strategies to test the antioxidant hypothesis in humans.