Activity decay of factor VIII in plasma at 37°C. Factor VIII (1 nM) was incubated at 37°C in factor VIII–deficient plasma and at the indicated times aliquots were removed and assayed using the one-stage clotting assays. Results are shown for WT (, ×), Asp519Ala (○), Asp519Val (●), Glu665Ala (△), Glu665Val (▲), Glu1984Ala (◇), and Glu1984Val (♦). Data were fitted by nonlinear least squares regression, and each point represents the value averaged from 3 separate determinations.