Thawed glycerolized red blood cells are washed in a system (1) of closed collapsible containers of flexible material which are positioned concentrically in a centrifuge rotor. The blood cells are held in an annular primary container (2) into which wash liquid is centrifugally fed from a c entral container (3) and from which supernatant is expressed into a central waste container (4) while the primary container is being compressed as a result of centrifugal action on an elastic body (24) in the rotor. A container assembly (1) for use in carrying out the washing comprises an annular collapsible primary container (2), a collapsible circular closed wash liquid container (3), a collapsible circular, closed waste container (4), and valve controlled conduits for passing liquid from the wash liquid container into the primary container and from the primary container into the waste container. The wash liquid container (3) and the waste container (4) are positioned one on top of the other in the circular area surrounded by the primary container (2).