Mononuclear cell-containing plasma separation composition and blood collection container

Provided is a mononuclear cell-containing plasma separation composition which is capable of suppressing the flow of a mononuclear cell-containing plasma separation composition during storage, is capable of satisfactorily forming partition walls during centrifugal separation, and is capable of obtaining mononuclear cell-containing plasma in which a small amount of blood cell components other than mononuclear cells are mixed. The mononuclear cell-containing composition for plasma separation according to the present invention contains an organic component having fluidity at 25 DEG C and two or more inorganic fine powders, and has a specific gravity of 1.060-1.080 (inclusive) at 25 DEG C. 本发明提供：能够抑制保存中的含单个核细胞血浆分离用组合物的流动，在离心分离时能够良好地形成隔壁，并且能够得到单个核细胞以外的血细胞成分的混入量较少的含单个核细胞血浆的含单个核细胞血浆分离用组合物。本发明的含单个核细胞血浆分离用组合物包含：在25℃下具有流动性的有机成分和2种以上的无机微粉，所述组合物在25℃下的比重为1.060以上1.080以下。