in centrifugal separation is however when the serum plasma separator has a specific gravity of less than 1 035 undesirably part of the serum plasma separator is sometimes separated into serum or plasma after the centrifugal separation the partition forming material used in the present invention can be selected from a wide range of materials including a variety of organic solvents having a low viscosity plasticizers and oily polymers oily polymers preferably having a viscosity of 200 to 600 000 cps at 25 u00b0 c can be properly used to achieve proper flowability and proper gelation in practical use examples of these oily polymers include silicone chlorinated polybutene chlorinated polybutadiene poly meth acrylate polyisobutene and a copolymer obtained from an u03b1 olefin or styrene and maleic acid diester the tackifier used in the serum plasma separator of the present invention is generally thermoplastic and has the form of a solid at room temperature it has a softening point of 70 u00b0 to 150 u00b0 c and a weight average molecular weight of approximately 200 to 1 500 when incorporated into an oily polymer the tackifier improves the adhesion and wettability to a blood separation tube when the softening point of the tackifier is lower than 70 u00b0 c the serum plasma separator is liable to flow in the blood separation tube at a relatively high temperature which may occur during the transportation thereof that is the storage stability of the serum plasma separator decreases and further hemocyte is often liable to remain in plasma or serum when the above softening point exceeds 150 u00b0 c the partition forming material has too high a viscosity for this reason the floatability of the partition forming material deteriorates and.