To determine the effects of hydrcxyethyl starch upon liver, kidney, serm electrolytes and particularly upon blood coagulation, 500 ml of 6% HES in saline solution was administered intravenously to 15 patients during elective minor surgery. In all cases weighted blood loss was less than 600 ml without replacement. The following laboratory tests were performed immediately before infusion and again 1 hour, 24 hours, 48 hours, and 1 week after the infusion: RBC, WBC, hemoglobin, hematocrit, ESR, platelet, bleeding time, coagulation time, prothrombin time, total protein, albumin, total bilirubin, direct bilirubin, SGOT, SGPT, alkaline phosphatase, BUN, sodium, potassium, and chloride. The results are as follows: 1) No anaphylactic shock or bleeding tendency characteristic of colloids was encountered. 2) No functional disturbance of liver or kidney directly attributable to HES was identified. 3) All laboratory parameters except WBC and ESR decreased after infusion. The decrease was, however, within the normal range and believed secondary to dilational effect of infusion. WBC increased somewhat, but returned to the preinfusion level in a week. 4) Exceptionally erythrocyte sedimentation rate increased notably during the 24 hour period following infusion. As with dextran, this was interpreted not due to direct effect of HES, but due to increased adsorption of fibrinogen, α-β-γ-glcbulin to red cell surface with changes in electric charge between red cells. 5) Clinical applicability, metabolic aspect, degree of subtitution of hydroxyethyl group and safety with multiple infusion of HES must be carefully determined.