Blood from a patient is treated extracorporeally by any selected one of a plurality of predetermined treatments. Each of the treatments involves the flow of blood from the patient into a primary chamber of a filtration unit, past a semipermeable membrane located in the filtration unit which separates the primary chamber from a secondary chamber of the filtration unit, out the filtration unit and back to the patient. A replacement fluid is selectively and controllably added to the blood, as required for the selected treatment. A secondary fluid is controllably and selectively introduced into the secondary chamber of the filtration unit for controllably collecting material passing across the semipermeable membrane from the blood or for supplying material to pass across the semipermeable membrane into the blood as required for the selected treatment. Materials are removed from the secondary chamber and collected in accordance with the selected treatment. Upon selection of one of the plurality of extracorporeal blood treatments available, the primary and secondary circuits are automatically primed with a priming fluid, in accordance with the selected treatment. An apparatus which automatically performs and monitors the selected treatment is also described.