In two groups, each consisting of five healthy volunteers, 7 ml blood/kg body weight were exchanged with equal amount of hydroxyethyl starch (HES) and dextran 60 solutions, respectively. Dextran 60 plasma levels, determined by the anthrone method, were undetectable after 4 weeks. The elimination of HES from the blood, determined by an immunological technique and by the anthrone method, had a very protracted course. Two weeks after infusion the HES plasma concentrations were 9% of the initial value and after 17 weeks they were still above the 1% level. The prolonged intravascular persistence of HES in its commercially available preparation, and the possibility of tissue accumulation after repeated HES infusions were considered undesirable. The hypothesis that HES infusion causes and augmentation of serum alpha-amylase concentrations in man was confirmed. This effect should be borne in mind when HES solutions are given to patients in whom the diagnosis of acute pancreatitis might be considered.