The one-stage clotting assay is the basis for the most widely used method for determining factor IX activity. It is based on the correction of the clotting time of fIX deficient plasma by the addition of fIX. Recently, a new assay for fIX has been developed that assay involves the addition of fIX to a preparation of fXIa and fX. The amount of fIX added can be determined from the rate of cleavage of a chromogenic substrate by factor Xa. We have undertaken a formal comparison of the clotting and chromogenic assays for determining potency of factor IX concentrates. Multiple comparisons of the assays using CBER's current fIX standard, FN2 versus the current WHO standard, 84/618, demonstrated equivalence of the two methods. The value of FN2 by clotting assay was 9.3 and 9.1 for the chromogenic assay. Comparisons of multiple lots of factor IX concentrates submitted to CBER for testing indicated significant disagreement between the methods for some concentrates. This disagreement could be somewhat reduced when the chromogenic assay was read kinetically rather than using the endpoint method. Further comparative studies are in progress.