The presence of free glucose in serum was found to interfere with accurate measurement, by a colorimetric method, of nonenzymatically glycosylated serum proteins. A mean elevation to 241% of basal levels was observed in the serum of 11 nondiabetic subjects to which glucose, in a concentration of 300 mg/dl, had been added immediately before assay. After dialysis of serum samples to remove glucose, levels of nonenzymatically glycosylated serum proteins were 0.27 ± 0.11 and 0.79 ± 0.24 nmol 5-hydroxymethylfurfural/mg protein (mean ± SD), respectively, in 57 nondiabetic and 62 type I diabetic subjects. Levels observed before dialysis of serum were approximately two to three times higher. These studies indicate that removal of free sugar from serum is necessary for accurate measurement of glycosylated protein by thecolorimetric method, and this can be achieved by overnight dialysis of serum against normal saline.