Low molecular weight (40,000) dextran is being extensively used in open heart surgery to minimize the amount of donor blood required and to inhibit red cell aggregation. At equal concentrations, its effects on blood coagulation were less than those of standard dextran plasma substitutes; but in the very high concentrations used in open heart surgery its effects were greater. Addition of low molecular weight dextran to normal plasma caused a gross precipitate and striking inhibition of coagulation. Factor VIII (antihemophilic factor) and fibrinogen were partially removed with the precipitate. Patients subjected to open heart surgery had lower levels of these two clotting factors in their plasma when dextran was used, but it is not certain whether this loss occurred in vivo, or during separation of the plasma from red cells (and precipitate) prior to testing. No proof was obtained in this series of cases that low molecular weight dextran increased the tendency to bleed, but the results indicate that this would be a likely possibility with the use of larger amounts.The low molecular weight dextran had a slight tendency to induce red cell aggregation by itself. It potentiated the aggregative effect of Polybrene and the agglutinating effect of a saline Rh antibody. On the other hand, it inhibited agglutination of red cells induced by an Rh antibody in a high protein medium. It thus appears likely that high concentrations of low molecular weight dextran in the blood of a patient could lead to erroneous conclusions in compatibility tests.