An isolated peptide useful as a selective antagonist of mammalian R-cadherin comprises 3 to 30 amino acid residues, three contiguous residues of the peptide having the amino acid sequence Ile-Xaa-Ser; wherein Xaa is an amino acid residue selected from the group consisting of Asp, Asn, Glu, and Gln. Preferably Xaa is Asp or Asn. In one preferred embodiment the peptide is a cyclic peptide having 3 to 10 amino acid residues arranged in a ring. The selective R-cadherin antagonist peptides of the invention are useful for inhibiting the targeting of stem cells, such as endothelial precursor cells, to developing vasculature, for inhibiting R-cadherin mediated cellular adhesion, and for inhibiting retinal angiogenesis.