Red blood cells are purified by defibrinating whole blood and then filtering the defibrinated whole blood, whereby at least a portion of a plasma component is separated from the red blood cells to form a suspension of red blood cells, thereby purifying the red blood cells. Whole blood is defibrinated by, for example, using a chemical coagulating agent or mechanical agitation. Separation of the plasma component from red blood cells can be completed by, for example, diafiltration. The suspension of red blood cells can then be employed to produce a hemoglobin-based oxygen carrier.