Gel-like composition useful for separating and partitioning whole blood into serum and clot portions. The composition has a specific gravity between that of the serum and clot portions such that, when centrifuged in the presence of whole blood, the composition forms a chemical and physical barrier between the serum and clot portions. The composition comprises, in combination, a silicone fluid, an inert siliceous filler dispersed therein, and a network former consisting of a polysiloxane-polyoxyalkyl copolymer which stabilizes the composition by minimizing and/or avoiding "wet out" of the siliceous filler with time. The composition has a viscosity within the range of about 200,000 to about 600,000 centistokes, preferably within the range of about 350,000 to about 450,000 centistokes.
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