The invention provides a serum or plasma separating composition which comprises a cyclopentadiene resin, a viscosity adjusting agent and an organic gelling agent. The composition is characterized in that the cyclopentadiene resin is a liquid resin at a temperature of 25.degree. C., the viscosity adjusting agent being a liquid ester obtained from propionic acid, butyric acid, benzoic acid or phthalic acid, the organic gelling agent being a condensation product of sorbitol and an aromatic aldehyde, the composition containing 0.8 to 25 parts by weight of the viscosity adjusting agent and 0.03 to 0.9 part by weight of the organic gelling agent per 100 parts by weight of the cyclopentadiene resin. The composition forms a satisfactory separating layer even when subjected to a centrifugal separation operation at a set temperature of 4.degree. C.