A spectrophotometric method for the assay of human blood coagulation factor VIII in plasma is presented. The chromogenic assay for factor VIIL·C in plasma is performed in 3 steps: (1) activation of factor VIII by thrombin; (2) activation of factor X in a mixture of factor X, factor IXa, phospholipids/Ca2+ and plasma containing activated factor VIII, and (3) determination of the rate of factor Xa formation with the chromogenic substrate S2337. Within-assay variation was between 5 and 6.9% for factor VIIL·C activities between 20 and 150%. Clotting and chromogenic factor VIIL·C activities were compared in plasma of 50 normal healthy donors (coefficient of correlation r = 0.8