A method of separating previously collected whole blood into red blood cells and plasma using a spinning membrane separator, in which the separator comprises a housing with a top and a bottom, at least one port (22, 34) adjacent each of the top and the bottom of the housing, and a porous membrane configured to spin about a generally vertically-oriented axis so that the surface of the membrane is oriented substantially vertically, which method comprises the steps of: - priming the membrane spinning separator comprising the steps of: - introducing a priming solution through the port (22; 34) at the bottom of the housing; - flowing additional priming solution through the port (22; 34) at the bottom of the housing so that a priming solution-air interface is formed that advances upwardly across the surface of the membrane through the housing to displace air within the housing and to expel the air through the port (34; 22) at the top of the housing, and to simultaneously wet the membrane; and - continuing to flow additional priming solution through the port (22; 34) at the bottom of the housing until the priming solution-air interface has advanced vertically across the entire membrane; - continuing operation to separate whole blood into plasma and red blood cells, wherein plasma is received in a plasma collection container and wherein red blood cells are received in a red blood cell product container (1 15); - rinsing the separator with a cell preservative solution at the end of the separation procedure, wherein during the solution rinse a plasma flow path leading to the plasma collection container is closed by a plasma clamp, so that any blood remaining in the system is flushed into the red blood cell product container. - priming the membrane spinning separator comprising the steps of: - introducing a priming solution through the port (22; 34) at the bottom of the housing; - flowing additional priming solution through the port (22; 34) at the bottom of the housing so that a priming solution-air interface is formed that advances upwardly across the surface of the membrane through the housing to displace air within the housing and to expel the air through the port (34; 22) at the top of the housing, and to simultaneously wet the membrane; and - continuing to flow additional priming solution through the port (22; 34) at the bottom of the housing until the priming solution-air interface has advanced vertically across the entire membrane; - introducing a priming solution through the port (22; 34) at the bottom of the housing; - flowing additional priming solution through the port (22; 34) at the bottom of the housing so that a priming solution-air interface is formed that advances upwardly across the surface of the membrane through the housing to displace air within the housing and to expel the air through the port (34; 22) at the top of the housing, and to simultaneously wet the membrane; and - continuing to flow additional priming solution through the port (22; 34) at the bottom of the housing until the priming solution-air interface has advanced vertically across the entire membrane; - continuing operation to separate whole blood into plasma and red blood cells, wherein plasma is received in a plasma collection container and wherein red blood cells are received in a red blood cell product container (1 15); - rinsing the separator with a cell preservative solution at the end of the separation procedure, wherein during the solution rinse a plasma flow path leading to the plasma collection container is closed by a plasma clamp, so that any blood remaining in the system is flushed into the red blood cell product container.