Assessment of vulnerable carotid artery plaques is a majorunresolved issue. Patients presenting unstable angina have evidenceof unstable carotid artery plaque [1] suggesting systemic vascularvulnerability [2].Neutrophil myeloperoxidase (MPO) is a powerful producer ofoxidant molecule [3]. MPO level is higher in patients with coronaryartery disease and in patients undergoing percutaneous coronaryintervention for unstable coronary syndrome compared to stablepatients [4,5], carrying predictive value of adverse outcome [6].MPO levels measured by immunological methods (ELISA or RIA)quantify the total concentration of the enzyme without providing itsenzymatic activity [7]. MPO activity is currently measured byspectrophotometry or fluorimetry [8,9]. However, these methodsare not applicable to complex biological samples such as humantissue. We recently developed a new technique, the SIEFED (SpecificImmunological Extraction Followed by Enzymatic Detection) tomeasure the activity of the enzyme in biological fluids and developedit for equine MPO and thereafter for human use [10,11]. The aims ofthis study were to assess the presence of the active form of MPO inhuman carotid atherosclerotic lesions, to compare active MPO withtotal MPO, and to assess the potential significance of total and/oractive MPO by comparing their levels in symptomatic and asymp-tomtic patients.We prospectively collected 90 carotid artery plaque specimensobtained by surgical thrombo-endarteriectomy according to theNASCET criteria [12].Carotid atherosclerotic plaques were surgically collected andrinsed with 0.9% NaCl solution before freezing at −20 °C. Methodol-ogy and validation of SIEFED technique as well as ELISA method havebeen previously described [7,10,11]. The activity of the total MPOexpressed in ng/mL was reported to the total protein concentration(mg/mL) measured in the supernatant of carotid extracts using the