The effect of dextran infusions upon the bleeding time was studied in 22 women and 235 men. In 42 % of the subjects the infusion of one liter of 6 % solution of dextran measurably prolonged the bleeding time; in 8% of the subjects the bleeding time after infusion exceeded 30 minutes. The effect was maximal from three to nine hours after the infusion and was not accounted for by any demonstrable dilution or reduction of fibrinogen or thrombocytes. Comparison of dextran preparations showed that those of highest molecular weight had the most marked effect in prolonging the bleeding time. The use of dextrans as plasma expanders carries a risk of serious failure of the hemostatic mechanism and is contraindicated in patients who are known to have a tendency to bleed or who have received large transfusions of whole blood.