Activation and inactivation of FVIII WT and^ARG531^HIS by thrombin. Immunoaffinity purified FVIII WT (□) and ^ARG531^HIS (•) proteins (0.5 nmol/L) were incubated with thrombin (1 U/mL) at room temperature and assayed over time for FVIII activity by aPTT. The results are from a single thrombin activation experiment and are typical of multiple independent experiments.