The article by Rock et al<sup>1</sup>in this issue ofThe Journalcalls attention to the problems of verifying the labeled potency of factor VIII concentrates. Rock et al report finding (in 1982) a considerable discrepancy between their measurements and the manufacturers-labeled factor VIII content, with the authors' assays showing overall levels of approximately 60% of the labeled values with each of four lots of one manufacturer's product. They point out the potential serious implications of such less-than labeled factor VIII content, including inadequate treatment of bleeding episodes with poor clinical response and increased factor VIII usage (at additional cost) if clinicians doubt that the bottle contains as much as stated on the label. This issue is not a new one and no doubt reflects several problems that have been of great concern to the manufacturers of clotting factor concentrates, the US Office of Biologics (OOB) and its British