Abstract: The rotor assembly is provided for the separation and harvesting of rare cells in a blood sample. A displacing fluid reservoir is located at least partially radially inward of a centrifuging chamber, wherein a displacing fluid of a greater density than the fluid in the centrifuging chamber is retained in the displacing fluid reservoir. Upon rotation of the rotor assembly, the displacing fluid is allowed to pass into the centrifuging chamber, typically at an outer periphery of the centrifuging chamber, so as to urge the latest separated component in the centrifuging chamber through an outlet port coincident with the axis of rotation of the rotor assembly.