Today the cryopreservation of human blood products is routine. However, before reinfusion the cryoprotectant, often glycerol, has to be removed. We have designed a combined microfiltration diafiltration process using microporous hollow fibres for removing glycerol from frozen red blood cell concentrates. As the system can be closed to the atmosphere there is no possibility of infection of the "washed" blood. Thus the post-thaw shelf life of the blood may be greatly increased. The process has been optimized by minimizing both the processing time and diluent volume required. Finally a hollow fibre module capable of completing the entire washing process in 30 min has been developed. We show that such a module requires hollow fibres with an inside diameter of 200 mu m. The design equations we present are generally applicable to the design of hollow fibre microfiltration systems.