Effect of vWF on fVIII binding to BO2C11. rfVIII (1 μg/mL) was incubated for 30 minutes at 37°C in the absence or presence of different concentrations of vWF before the addition to BO2C11-coated plates. The binding of fVIII to BO2C11 was detected by the addition of the anti-A2 MoAbF15B12 after 5 minutes of incubation at 37°C. Bound fVIII was calculated by comparison with a calibration curve established with known levels of rfVIII bound to BO2C11 (see the Materials and Methods).