The IBM 2991 Blood Cell Processor, normally used to wash or deglycerolize red blood cells, has been modified to permit serial plasmapheresis using a small roller pump, an adapter line and the disposable processing set usually used for cell washing. Blood is withdrawn from the donor, pumped into the plastic centrifuge bag and the plasma removed after centrifugation. Donor erythrocytes are pumped from the processing bag to a reservoir bag from which they are returned to the donor by gravity. Multi-unit plasmapheresis to harvest plasma for transfusion or to remove plasma as a therapeutic measure is possible using the modified system. A four-unit plasmapheresis can be done in 65 to 75 minutes at a cost of $27.61, including disposables and technologist labor. This modification allows the IBM 2991 to function for red blood cell washing and as an efficient instrument for plasmapheresis.