One means by which oxidation of low density lipoproteins (LDL) may contribute to atherogenesis is by their ability to induce the release of interleukin-1 beta from monocytes. In the present study, the effect of the lipophilic nitrone spin trap alpha-phenyl-tert-butylnitrone (PBN) on lipoprotein oxidation and subsequent release of interleukin-1 beta was examined. The hydrophilic nitrone spin trap alpha-(4-pyridyl 1-oxide)-N-tert butylnitrone (POBN) was also studied to evaluate the importance of spin trap localization within the lipoprotein. PBN inhibited copper-induced modification of low density lipoproteins in a dose-dependent fashion as judged by measurement of thiobarbituric acid reactive substances, electrophoretic mobility, and fluorescence changes, while POBN was relatively ineffective. As demonstrated by electron spin resonance spectrometry, spectra of PBN adducts were highly immobilized, which reflects their presence within the LDL matrix. Experiments using chromium oxalate, a paramagnetic relaxing agent, revealed that the PBN adduct is composed of a mobile component (exposed to the LDL aqueous phase) and an immobilized component, localized in the lipid-protein interface or in the bulk lipid. Cholesteryl ester phospholipid dispersions in which only the core cholesteryl esters are subject to oxidation were also used to compare PBN and POBN. Again, PBN prevented the oxidation of the core lipids while POBN had little effect, further suggesting that PBN is capable of localization within the core of LDL. In agreement, attenuation of low density lipoprotein oxidation by PBN also decreased their ability to induce interleukin-1 beta release from human monocytes. Conversely, POBN had little effect on the release of interleukin-1 beta from cells incubated with copper-oxidized lipoproteins. These results suggest that the ability of a nitrone spin trap to prevent low density lipoprotein oxidation and its biologic effect(s) requires its incorporation into the particle.