Tumor blood vessels can be selectively targeted by RGD-peptides that bind to alpha(v)beta(3) integrin on angiogenic endothelial cells. By inhibiting the binding of these integrins to its natural ligands, RGD-peptides can serve as antiangiogenic therapeutics. We have prepared multivalent derivatives of the cyclic RGD-peptide c(RGDfK) by covalent attachment of the peptide to side chain amino groups of a protein. These RGDpep-protein conjugates inhibited alpha(v)beta(3)-mediated endothelial cell adhesion in vitro, while conjugates prepared with a control RAD-peptide showed no activity. Radiobinding and displacement studies with endothelial cells demonstrated an increased affinity of the RGDpep-protein conjugates compared to the free peptide, with IC50 values ranging from 23 to 0.6 nM, depending on the amount of coupled RGDpep per protein. Compared to the parental RGD-peptide and the related RGD-peptide ligand c(RGDfV), the RGDpep-protein conjugates showed a considerable increase in affinity (IC50 parent RGDpep: 818 nM; IC50 c(RGDfV): 158 nM). We conclude that the conjugation of RGD-peptides to a protein, resulting in products that can bind multivalently, is a powerful approach to increase the affinity of peptide ligands for alpha(v)beta(3)/alpha(v)beta(5) integrins.