Oxidative modification of low density lipoproteins (LDL) has been implicated in the sequence of events leading to fatty streak formation in the arterial intima. Increased oxidative modifications of dense versus buoyant LDL particles could contribute to increased atherosclerosis associated with lipoprotein profiles enriched in small, dense LDL. In the present studies, we compared rates of copper-induced oxidative changes for six LDL subfractions ranging in density from 1.023 to 1.053 g/ml and mean particle diameter from 282 +/- 10 to 245 +/- 3. Rates of formation of thiobarbituric acid-reactive substances (TBARS), as indicated by the time required for half-maximal TBARS formation (T1/2max), decreased with increasing density and decreasing particle diameter to a minimum in fraction 5 (d = 1.046 g/ml, diameter = 250 +/- 5) (P = 0.007). In parallel studies using unfractionated LDL (d = 1.019-1.063 g/ml), T1/2max values were inversely correlated with the predominant LDL species diameter as determined by 2-16% gradient gel electrophoresis (P less than 0.05), consistent with the involvement of subclass composition in determining oxidative behavior. In separate experiments, subfraction differences in oxidation rates as assessed by TBARS formation were verified by the finding of similarly dispartate changes in fluorescence intensity and anionic electrophoretic mobility. T1/2max values were not related to LDL contents of alpha-tocopherol, beta-carotene, protein, triglycerides or phospholipids, but were significantly correlated with unesterified cholesterol content (r = 0.46; P less than 0.001) and were inversely associated with cholesteryl ester content (r = 0.28; P less than 0.05). The positive association of T1/2max with unesterified cholesterol suggests that this constituent may impart resistance to oxidative modification, possibly by altering properties of the surface monolayer where it resides.