The development of a novel manual method designed to measure serum glycosylprotein as an index of diabetic control is described. The method relies on the ability of ketoamines (fructosamines) to act as reducing agents in alkaline solution. Conditions are described for a simple colorimetric procedure which permits assay of both a synthetic fructosamine and purified albumin while severely limiting the contribution of interfering substances. Applied to whole sera, the measurement is linear with volume of serum assayed. It allows clear discrimination of normal and diabetic populations (p less than 0.001), and is significantly correlated with fasting blood glucose concentration (r = 0.72) and with a thiobarbituric acid procedure for measuring glycosylprotein-derived hydroxymethylfurfural (r = 0.58). The method is rapid (at least 12 samples per hour) and demands only simple equipment.