The RGD-containing peptides isolated from the venoms of the Viperidae constitute a new class of small cysteine-rich peptides of variable amino acid composition and biological activity (Huang, T.-F., et al. (1987) J. Biol. Chem. 262, 16157-16163; Gan, Z.R., et al. (1988) J. Biol. Chem 263, 19827-19832; Huang, T.-F., et al. (1989) Biochemistry 28, 661-668), which it is proposed by Gould et al. (unpublished data) that we call 'disintegrins'. These peptides bind to the glycoprotein IIb-IIIa receptor on the platelet surface and inhibit aggregation induced by ADP, thrombin, platelet-activating factor and collagen. These peptides are also potent inhibitors of cell adhesion to fibrinogen (Knudsen, K.M., et al. (1988) Exp. Cell Res. 179, 42-49). We report the isolation of two further RGD-peptides from the venoms of Trimeserusus elegans and Trimeserusus albolabris, purified to homogeneity with high yield by a novel, rapid reverse-phase HPLC method. The primary structures of these two peptides were determined to be single polypeptide chains of 73 amino acids. Albolabrin differed from trigramin by eight residues whilst elegantin differed by 22 residues. The molecular mass of albolabrin calculated on the basis of amino acid sequence was 7574 Da and the pI similarly calculated was 4.27. The molecular mass of elegantin was calculated to be 7806 Da and the theoretical pI to be 4.69. RGD is maintained in the same position (51-53 AA) and all 12 cysteines are identical. Our data suggest that the presence of RGD, the conserved secondary and tertiary structure, are essential for the expression of biological activity by these peptides. Both peptides inhibited ADP-induced platelet aggregation. Extended homologies around the RGDS sequences in human von Willebrand Factor and bovine fibrinogen were found with both peptides.