Factor VIII (FVIII) is assayed by one-stage and two-stage clotting methods and by chromogenic methods, although the chromogenic method has largely replaced the two-stage clotting assay. Clinical plasma samples are assayed mostly by one-stage assays, but most manufacturers of concentrates use the chromogenic method, which is more precise and is the reference method of the European Pharmacopoeia and the International Society on Thrombosis and Haemostasis (ISTH). For most plasma-derived concentrates, assays against the World Health Organization (WHO) concentrate standard give similar results with the one-stage and chromogenic methods, but for products produced by the "method M" monoclonal antibody process, the one-stage potency is 25 to 30% higher than the chromogenic potency. For full-length recombinant products assayed against a plasma-derived concentrate standard, one-stage potencies are about 10% lower than chromogenic potencies, but for the B-domain deleted recombinant product ReFacto, the discrepancy is larger-from 20 to 50%. These discrepancies emphasize the need for an international methodology for labeling of concentrates. In ex vivo assays of hemophilic plasmas after infusion of concentrates, large discrepancies are found among laboratories and with different assay methods when a plasma standard is used. In most studies, the chromogenic potencies are higher than the one-stage potencies, and the discrepancy is highest for recombinant products. This discrepancy can be largely eliminated by the use of concentrate standards, diluted in FVIII-deficient plasma, to assay postinfusion plasma samples.