A method is disclosed for priming a kit for use in a therapeutic apheresis procedure with previously-collected blood prior to flowing the patient's whole blood into the inlet line of the kit, as part of a procedure in which a selected blood component is separated from a patient's whole blood, and replaced with a previously-collected blood component. The operator enters into the controller of the separation device the hematocrit value of the previously-collected blood. Then, in response to prompts by the controller, the operator enters a target hematocrit value for the previously-collected blood and an identification of the portion of the kit to be primed. The identified portion of the kit is automatically primed with the previously-collected blood. The patient is then connected to the inlet line of the kit in response to a prompt from the controller and the therapeutic procedure is commenced.