Increasing opportunities of applying heart-lung machine for open heart surgery, demands reducing an amount of whole blood by the addition of priming substituent. Among hemodiluents which have been studied, low molecular weight dextran (LMD) was reported to have a flow-improving effect in microcirculation by preventing or inhibiting the intravascular aggregation of red blood cells. To establish some theoretical support for this advantageous hemodilution by LMD, the author has performed clinical and experimental studies, particularly investigating mechanism of antisludging effect of LMD, and the result was summarised as follows. In hemodilution by LMD, uniform increase in electric negativity of both red blood cell and platelet was consistently observed in vitro, as well as improvement of suspension stability of blood. However, addition of each of glucose, polyvinylpyrrolidon, mannit, sorbit and amino acid to the whole blood as a diluent failed to cause any significant change in electric charge of blood cell, while improvement of suspension stability was obtained. In extracorporeal perfusion with whole blood as the priming vehicle, reduction of negative charge of blood cell was occurred. The increased negativity of blood cell in the presence of LMD was gradually decreased during perfusion, but it was kept higher than that before LMD was added, and this gave apparent benefit to the clinical extracorporeal circulation. LMD also decreased and normalized a positive charge in injured intima of blood vessel, which may probably attract negatively charged blood corpuscles, and this effect of LMD may prevent subsequent development of intravascular thrombosis. Addition of LMD to the whole blood has prevented both the decrease in electric negatitivy of blood cell and the increase in viscosity of blood and plasma, which were always observed during hypothermic procedures. Surveying the biochemical character of polimer, disadvantage in reduction of electrical negativity of the blood cell by addition of either polybrene or protamine was discussed, and possible future improvement of heparin neutralization was anticipated.