Abstract: Blood separation is accomplished by alternately extracting blood from and reinfusing blood into a donor by way of a single needle while simultaneously and continuously separating the extracted blood into constituents. A harness set applicable to a hemapheresis instrument includes a dual-compartment reservoir. A first compartment stores blood during extraction for supply to the separator during reinfusion. The second compartment stores packed cells during collection for return to the donor during reinfusion. The collection and reinfusion cycles are alternated rapidly to preclude clotting in the un-anticoagulated portion of the harness tubing between the venepuncture needle and the Y-connection with the anticoagulant line.