Provided is a composition for mononuclear cell-containing plasma separation capable of suppressing a flow of the composition for mononuclear cell-containing plasma separation during storage, capable of satisfactorily forming a partition during centrifugation, and capable of yielding mononuclear cell-containing plasma with little admixture of blood cell components other than mononuclear cells. The composition for mononuclear cell-containing plasma separation according to the present invention contains an organic component having fluidity at 25° C. and two or more kinds of inorganic fine powders, and has a specific gravity of 1.060 or more and 1.080 or less at 25° C.