vWF is a multimeric glycoprotein that serves as the major carrier in plasma of Factor VIII (FVIII). We have used an anti-human vWF MAb W5-6A to investigate the FVIII binding site on vWF. W5-6A inhibited FVIII binding to vWF-coated polystyrene tubes in a concentration-dependent manner with 90% inhibition of FVIII binding at a concentration of 10 micrograms/ml. The W5-6A epitope was identified by screening a vWF fragment library using the bacteriophage expression vector lambda gt11. DNA sequence analysis of 29 immunoreactive phage clones localized the W5-6A epitope to a nonadecapeptide spanning amino acid residues threonine 78 to threonine 96 at the amino-terminus of the mature vWF polypeptide. Purified beta-galactosidase/vWF fusion protein from one of these clones, vWF9, was incubated with radiolabeled W5-6A and caused near complete inhibition of W5-6A binding to vWF. Inhibitory activity was lost after vWF9 trypsinization or reduction and alkylation. These data indicate that (a) the antigenic determinant recognized by W5-6A localizes to a nonadecapeptide at the NH2 terminus of the mature vWF polypeptide, (b) disulfide bonds within vWF9 may be necessary to maintain the structure required for immunoreactivity with W5-6A, and (c) W5-6A recognizes an immunogenic region on vWF that may be at (or near) the major FVIII binding domain.