The existing standard for the measurement of activated blood coagulation factor X (FXa) is a non-WHO Reference Material (75/595) available from the National Institute for Biological Standards and Control (NIBSC). This bovine FXa reference material was not formulated and filled by NIBSC, rather it was prepared and freeze-dried into sealed ampoules in 1975 by an external group, organised by Jackson et al., and arbitrarily assigned a potency of 1.0 unit per ampoule [1]. With no information on the uniformity of the fill of 75/595, or on its stability, the suitability of the continued use of this reference material was questioned and a replacement for 75/595 was investigated. This article is protected by copyright. All rights reserved.