Nonenzymatic posttranslational glucosylation of the free amine of lysine residues can occur in hyperglycemic diabetic subjects. Using monoclonal antibodies that specifically bind the reduced conjugate of glucose covalently bound to the epsilon amine of lysine, glucitollysine, plasma lipoprotein glucosylation was demonstrated by radioimmunoassays in all subjects tested. Lipoproteins isolated from the plasma of diabetic subjects in poor metabolic control contained up to 33-fold increases in glucitollysine residues/mg of isolated lipoprotein protein, and on an absolute basis contained between 36 and 383 nmol of glucitollysine in their total lipoprotein fraction compared with normals, who had a mean of 2.9 ± 0.06 nmol. The majority of the glucosylated protein in the d < 1.125 g/ml fraction was present in the triglyceride-rich lipoproteins of hyperglycemic subjects, whereas the majority of the glucosylated protein in the d < 1.125 g/ml fraction of euglycemic subjects was present in the high-density lipoproteins (HDL). A number of immunochemical approaches were used to demonstrate that, in diabetic plasma, apo Al, apo All, apo B, apo Cl, apo E, and albumin were glucosylated. Detailed studies of the apoproteins of a d ≤ 1.019 g/ml lipoprotein fraction and of an HDL fraction isolated from a hyperglycemic diabetic subject indicated that glucitollysine-specific antibodies can be sed to preparatively isolate proteins containing glucosylated amino acid residues so that the extent of glucosylation in specific proteins can be measured. The results indicate that some of the lysine residues of the apoproteins can be modified in hyperglycemic diabetic subjects by the covalent attachment of glucose. The possible significance of these observations is discussed.