An improved design for fixed angle quantity type rotors is presented in which the holes are drilled at 10° to the axis of rotation and the Lusteroid tubes containing the centrifugate are individually sealed by means of tapered plugs. The advantages of this design are (a) increased capacity for a given diameter rotor, (b) prevention of tube collapse at all speeds and (c) more efficient concentration of materials in solution. Some charts illustrating differential sedimentation and concentration of proteins are given.