A fully enzymatic method for the determination of inulin in serum or plasma without deproteinization is described. The assay is carried out by means of a fructose determination after hydrolysis of inulin via inulinase and simultaneous oxidation of the native glucose using glucose oxidase and H2O2. This time-saving procedure of high specificy, sensitivity and accuracy requires only small sample volumes. The usefulness of the method is shown by analysis of blood levels in humans and rabbits after an intravenous administration of Inutest®.