The fructosamine test is considered clinically useful for assessing short-term integrated control of blood glucose, but there are few published data to support this hypothesis. We fractionated glycated and nonglycated proteins by affinity chromatography on phenylboronate columns and, with specific immunochemical methods, determined in the eluted fractions the following proteins, selected according to their biological half-lives and relative concentrations in serum: albumin, IgA, IgG, IgM, apolipoprotein B, haptoglobin, transferrin, alpha 1-antitrypsin, and alpha 2-macroglobulin. We found the following correlations between fructosamine (mmol/L) and, respectively, glycated albumin, IgG, and (albumin + IgG) (each in grams per liter): r = 0.901, 0.702, 0.878. IgM had the highest percentage of glycated molecules (range 11.1-37.5%, mean 22.4%), haptoglobin and alpha 1-antitrypsin the least. This result was almost independent of the proteins' molecular masses and fractional catabolic rate. Albumin evidently contributes most to results of the fructosamine test, confirming conclusions obtained in different ways by others.