Effect of GP Ibα MoAb and heparin on ³⁵S-methionine labeled vWF-A1 binding to HUVEC. ³⁵S-vWF–A1 was added to confluent HUVEC monolayers in microtiter wells and incubated in the presence or absence of a 1:125 dilution of the anti-GP Ibα MoAbs AS-7, Ib1, or 6D1, or 50 μg/mL of heparin. After incubation at 4°C for 30 minutes and repeated washes, bound ³⁵S-vWF–A1 was detected by counting HUVEC lysates. Data are expressed as vWF-A1–specific counts.