Results of assays of recombinant FVIII concentrates have been reviewed over a 10-year period. Initially there was wide variability between laboratories but this was minimised by the development of standardised assay methodology, in particular the use of haemophilic plasma for pre-dilution and 1% albumin in assay buffers. Using this standardised methodology and concentrate standards, there were no major differences in potency between one-stage, two-stage and chromogenic assays on the two full-length recombinant FVIII concentrates. However, using a plasma standard, the chromogenic method gave much higher potencies than the one-stage method on the same concentrates, and this explains a similar discrepancy found in patients' post-infusion samples after injection of recombinant concentrates. It is suggested that concentrate standards be used for such post-infusion samples in order to minimise this discrepancy.