A chromogenic factor Xa generation method has been developed for comparing the co-factor activity of factor VIII concentrates at physiological factor VIII concentrations (1 iu/ml). In the presence of thrombin all concentrates gave similar rapid rates of factor Xa generation, but in the absence of thrombin there were major differences between the rates of Xa generation between different products. High purity products, particularly those prepared by monoclonal antibody purification from plasma and recombinant sources, gave more rapid Xa generation than most intermediate-purity products. There was a very strong correlation between the rate of Xa generation and the difference in factor VIII potency by one-stage and two-stage assays. These results suggest the possible presence of small amounts of activated factor VIII in some concentrates, but differences in von Willebrand factor content could also contribute towards the different rates of factor Xa generation observed.