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The National Blood Service has performed an operational evaluation of the Pall Prion Reduction Filter (PRF1BU) on behalf of the UK and Ireland Prion Reduction Working Group. The filter is a device for use on red cells and requires sterile connection to the red cell component. The evaluation took the form of three individual pieces of work which included: (i) an evaluation of in vitro immunohaematological effects on red cells following prion filtration; (ii) an operational assessment of the routine use of the Pall prion reduction filter within a blood processing department; and (iii) an evaluation of the storage characteristics of red cells after prion reduction, including irradiated and washed red cells. The results of these evaluations indicate that, with the exception of a significant loss of haemoglobin caused by retention of red cells in the prion filter, the process of filtration through the PRF1BU does not appear to be detrimental to either the functional or immunological characteristics of red cells as discernible from in vitro testing. It is clear from these results however, that prion reduction of red cells derived from ‘Top and Bottom’ (TAB) collection packs yields a final red cell component that will not comply with the EU Directive requirements for 40 g haemoglobin per pack. This is because the normal processing of TAB packs requires the removal of a buffy coat that directly reduces the haemoglobin content of the final red cell component. Prion reduced red cell components derived from ‘Top and Top’ (TAT) systems do not undergo buffy coat removal and prion filtered red cells produced from such starting material are able to comply with this EU Directive specification. In addition, 60% of prion reduced, thawed and washed TAT red cells did not comply with the EU Directive specification for haemoglobin content (36 g per pack).