Abstract: The invention relates to devices, methods and kits for separating biological materials, particularly populations of cells of different densities. The invention finds particular utility in the separation of blood into its different component parts or cellular populations. An insert for a centrifuge tube is disclosed that facilitates the effective separation of blood fractions by achieving a sharp and distinct interface between the density gradient medium and the blood sample. Methods and kits involving the use of the insert in separating cells are described.