PURPOSE: To separate easily and effectively a serum or plasma from blood corpuscles without contamination, by using a sealant consisting of a thixotropic gelatinous material obtained by dispersing an organic gelling agent and inorganic fine powder in a fluid acrylic resin.

CONSTITUTION: The titled sealant for separation of a serum or plasma is characterized by dispersing an organic gelling agent, and additionally an inorganic fine powder according to the need, in a fluid acrylic resin to form a thixotropic gelatinous material, moreover allowing the density to be 1.03W1.08, the viscosity 1,500W7,000ps at 25°C. The main component fluid acrylic resin is consisting of a polymer obtained by polymerization of ≥1 functional or nonfunctional alkyl acrylate or alkyl methacrylate as a polymerizable monomer. As the organic gelling agent, for exapmle, "Gelol " is used, and as the inorganic fine powder used for a density modifying additive, materials of high density and inactivity such as silicon dioxide, alumina, titanium dioxide, calcium carbonate are preferable.