Abstract: A blood separating composition for use in blood collection tubes, the composition having an absolute viscosity of 100,000-320,000 cP at 25.degree. C. at a shear rate of 1 sec.sup.-1 when the blood separating composition turns into its dynamic state from its stationary state, with a reduced viscosity of 50,000 cP or more at a shear rate of 10 sec.sup.-1 in the dynamic state. The absolute viscosity of said composition is 100,000-300,000 cP at the shear rate of 1 sec.sup.-1 when returning to the stationary state. Such a viscosity renders stable and less flowable and blood separating composition during storage, allowing it to form a stable partition barrier in each collection tube when centrifuged to separate the blood phases one from another, with the blood cells above the barrier being prevented from remaining within the serum. The blood separating composition does not release any harmful oily substance or the like.