The binding of von Willebrand factor (vWF) to platelet glycoprotein (GP) Ib receptor is one of the initial events in thrombus formation. Previous studies have shown that RG12986, a reduced and alkylated recombinant fragment of vWF (Ser445-Val733), can inhibit binding of native vWF to GP Ib and offers potential as an anti-thrombotic agent. We have now evaluated a series of deletion mutants of RG12986 and found that reduced and alkylated rvWF508-704 is close to the minimal sequence with optimal RG12986-like activity (IC50 for inhibition of GP Ib-dependent platelet aggregation in the absence of modulators: 0.022 microM +/- 0.01, n = 3) and that it too binds directly to GP Ib. Under in vitro conditions, with no exogenous modulators present and in the absence of shear stress, oxidized rvWF508-704 (containing a disulfide bond between Cys508 and Cys659) is approximately 5-fold less active than reduced and alkylated rvWF508-704; the two fragments, however, display comparable activity in the presence of the modulator botrocetin. The smaller rvWF508-704 fragment offers distinct advantages over RG 12986. In particular, removal of non-active NH2 and COOH terminal sequences may reduce the risk of antigenicity and may contribute to rendering the molecule mostly monomeric in solution, as opposed to the monomer-dimer equilibrium previously described for RG12986.