It is well established that radical reaction of low density lipoprotein (LDL) causes fragmentation and cross-linkage of apolipoprotein B-100 (apoB). Our previous studies demonstrated that fragmented and cross-linked apoB proteins are present in normal human serum and tended to increase with age based on immunoblot analysis. These observations suggest that the fragmentation and cross-linkage pattern of apoB reflects the oxidative stress in an individual and that this pattern is a good atherosclerotic index. In this study, a method was developed to evaluate the fragmentation and conjugation pattern of apoB. A parameter named B-ox was introduced for each serum sample to quantitate the staining bands of the immunoblotting analysis. B-ox represents the relative abundance of radical reaction products (a sum of fragmented and conjugated apoB proteins) based on one control subject. If this value increases, it indicates that radical reaction products have increased, i.e., the oxidative stress has increased in the subject. Based on measurements of subjects in a rural area of Japan, B-ox showed significant positive correlation with intima-media thickness (IMT) of the carotid artery, LDL cholesterol, and age, while it showed significant negative correlation with high density lipoprotein (HDL) cholesterol and vitamin C. These results suggest that B-ox is a reliable indicator of atherosclerosis.