PURPOSE: To keep serum and blood corpuscles unchanged even allowing them to stand separated from each other by employing a blood separation agent mainly composed of polyester polyurethane which is about 20W50,000 poise in the viscosity and 1.015W1.075g/cm3 in the density at the temperature of 20°C.

CONSTITUTION: 1,000 part of 1,2-polypropylene glycole wth the average molecular weight of 4,000 and 42 part of diphenyl methanediisocyanate are put into an autoclave and caused to react under a current of nitrogen at 160°C with an agitation for 6hr under a normal pressure. 100 part of polyeter polyurethane is mixed with 8 part of hydrophobic aerosil to obtain a thixotropy material with a specific gravity of 1.051 and a viscosity of about 20,000 poise. After heated up to 50W70°C, the composition is distributed into 10cc serum separation blood sampling tubes and sealed up follwing the vacuum process to remove bubbles. After the sampling with the sampling tubes, a centrifugal separation forms a barrier layer between the serum and the blood corpuscles. This barrier won't be destroyed at all even when it is left alone for several days.