Glycation levels in serum apo B proteins were measured by colorimetry of fructosamine using nitroblue tetrazolium in immunoprecipitates generated using anti-apo B serum. The serum levels of apo B glycation determined by the proposed method correlated well with the fructosamine concentrations in low-density lipoprotein (LDL) fractions separated by ultracentrifugation. Close correlations were also observed with serum concentrations of fasting blood glucose, total cholesterol and apo B protein, but those with serum triglyceride, fructosamine and HbA1 c were weak. The normal reference value was 18.1±3.65μmol/l and inter-and intra-assay coefficients of variation were 4.6-6.1% and 5.9-6.6%, respectively. Diabetes mellitus patients with vascular complications had higher serum glycated apo B levels than those without complications, matched for risk factors such as blood pressure, obesity, smoking history, age, sex, duration of diabetes, serum total cholesterol, triglyceride, high-density lipoprotein (HDL) cholesterol and lipoprotein (a). High levels of glycated apo B proteins in the blood might be an independent risk factor in addition to the established risk factors for diabetic vascular complications.