Atherosclerosis is associated with increased lipid peroxidation, leading to generation of multiple oxidation-specific epitopes (OSEs), contributing to the pathogenesis of atherosclerosis and its clinical manifestation. Oxidized cholesteryl esters (OxCEs) are a major class of OSEs found in human plasma and atherosclerotic tissue. To evaluate OxCEs as a candidate biomarker, we generated a novel mouse monoclonal Ab (mAb) specific to an OxCE modification of proteins. The mAb AG23 (IgG1) was raised in C57BL6 mice immunized with OxCE-modified keyhole limpet hemocyanin, and hybridomas were screened against OxCE-modified BSA. This method ensures mAb specificity to the OxCE modification, independent of a carrier protein. AG23 specifically stained human carotid artery atherosclerotic lesions. An ELISA method, with AG23 as a capture and either anti-apoAI or anti-apoB-100 as the detection Abs, was developed to assay apoAI and apoB-100 lipoproteins that have one or more OxCE epitopes. OxCE-apoA or OxCE-apoB did not correlate with the well-established oxidized phospholipid-apoB biomarker. In a cohort of subjects treated with atorvastatin, OxCE-apoA was significantly lower than in the placebo group, independent of the apoAI levels. These results suggest the potential diagnostic utility of a new biomarker assay to measure OxCE-modified lipoproteins in patients with CVD.