A heterodimeric disintegrin designed as lebein was isolated from crude Vipera lebetina venom using gel filtration, anion and cation exchange chromatographies on FPLC. The amino acid sequence of each subunit determined by Edman degradation contains 64 residues with ten half-cystines and an RGD site at the C-terminal part of the molecule. The molecular mass of native lebein determined by mass spectrometry was found to be 14 083.4 Da and those of α and β subunits were 6992.05 and 7117.62, respectively. These value are in good agreement with those calculated from the sequences. This protein strongly inhibits ADP induced platelet aggregation on human platelet rich plasma with IC50=160 nM. Sequences of this protein subunits displayed significant sequence similarities with many other monomeric and dimeric disintegrins reported from snake venoms. We identified an amino acid residue (N) in the hairpin loop of both subunits (CRARGDDMNDYC) which is different from all other reported motifs of disintegrins and this subtle difference may contribute to the distinct affinities and selectivities of this class of proteins.