The method involves separating whole blood into blood components by centrifugation, and supplying one separated component into a primary bag (3). The component is transferred from the primary bag into a component bag (7), and is mixed with an additive solution. The component bag is separated into two compartments having variable volumes of blood component and solution, respectively. The additive solution is extracted from the latter compartment and is mixed with the component, and the mixture of the component and four additive solutions is transferred into the former compartment. An independent claim is also included for a blood processing device.
