Recent use of hetero-dimerization to improve the affinity of peptide ligands has made peptides an attractive alternative to small molecules and proteins. Using this strategy, we have developed peptides with affinities comparable to antibodies and specificities often better than small molecules or antibodies. These peptides can be used as a delivery vehicle for drugs or diagnostics, especially in the case of tumor targeting cytotoxic drugs or targeted diagnostics. We describe here an assay to identify an ideal pair of peptides suitable for heterovalent ligands.