By radioimmunoassay, we achieved to quantitate nonenzymatically glucosylated human serum proteins. Antiserum was obteined by immunizing guinea pig with reduced glucosylated human albumin (RedGlc-HA), and we used the antiserum for radioimmunoassay after absorbing the antibody against native human albumin. Using polyethylene glycol, we measured antibody respons by the degree of 125I-RedGlc-HA bound to this antiserum. As the result, RedGlc-HA and ε-N-1-(1-deoxy glucitol) lysine (glucitollysine) were efficient competitor for the antibody. Reduced glucosylated guinea pig albumin (RedGlc-GPA), in which glucitollysine was present, was also an efficient competitor for this antibody. So it seemed that glucitollysine was an important component of antigenic region for this antiserum. Using this antiserum, we quantitated reduced glucosyated protein (RedGlc-Prot) in sodium borohydryde-reduced human sera, by measuring the degree of their abilities of binding this antiserum, comparing the ability of glucitollysine as a standard. As the result, RedGlc-Prot was present in the sera, obtained from sixteen diabetics, in significantly larger quantity than in those obtained from eight normal subjects. (121.6±29.2 nmol/mg·prot vs 63.8±9.6, p.<0.001).