It has been known that the echogenecity of unclotted blood under stasis is induced by mixture of red bloodcell and fibrinogen and it is changed by hematocrit, temperature, fibrinogen concentration and hemolysis. Authoranalyzed the echogenecity of human unclotted static blood in vitro with changing temperature, hematocrit, plasmafibrinogen concentration and also the echogenecity of hemolyzed blood and pure plasma. The results were asfollows; 1. As the temperature was a increased, the echogenecity of the blood was increased. 2. After hemolysis ofthe blood with saponin, the echogenecity of blood was scanty or nearly absent despite of the changes oftemperature. 3. As the hematocrit was increased, the echogenecity of the blood was increased. 4. As thefinbrinogen concentration was increased, the echogenecity of the blood was increased. 5. No significantechogenecity was found in the plasma.