In the present study, both electron spin resonance (ESR) and chemical detection confirmed that lutein [extracted from alfalfa (Medicago sativa L.)], the most abundant xanthophyll in thylakoids of chloroplasts, could serve as an antioxidant to scavenge reactive oxygen species (ROS) in vitro. Lutein exhibited a greater capacity for scavenging hydroxyl (OH^·) and superoxide (O₂^·–) radicals than β-carotene at the same concentration, whereas the opposite trend was observed in the capacity for scavenging singlet oxygen (¹O₂). The capacity of lutein for scavenging ROS from high to low is OH^· > O₂^·– > ¹O₂. We hypothesise that lutein plays an important photoprotective role in scavenging O₂^·– and OH^· under severe stress. This hypothesis is consistent with our previous report that the lut2 (lutein-deficient) Arabidopsis mutant is more susceptible to damage than the npq1 (lutein-replete but violaxanthin de-epoxidase-deficient) Arabidopsis mutant under severe stress during exposure to high light intensity at low temperature (Peng and Gilmore 2003).