The development of successful, targeted therapies would benefit from a methodology that enables the rapid discovery of multiple ligands in a cost effective manner. As a result, the identification of specific targeting agents binding to unique or over-expressed receptors could greatly enhance drug potency. To discover novel tumor targeting ligands, a methodology using bacterial surface display was developed to isolate peptide sequences that bind with high specificity to several breast carcinoma cell types through alternating positive selections on tumor cells and negative selections on normal human breast epithelial cells. To anchor peptide libraries to the surface of E. coli, an outer membrane protein was circularly permuted to allow for peptide fusions to an extracellular terminus. Libraries were constructed by creating a collection of bacteria that each independently express a unique peptide, either a fully random 15mer (X₁₅) or a constrained 7mer (X₂CX₇CX₂). An intracellular fluorescent protein was also expressed in the bacteria allowing for facile screening of peptide libraries binding to tumor cells through the use of quantitative fluorescence activated cell sorting (FACS). To assay peptide function outside of the display context, tandem repeats of the targeting peptides were constructed in order to retain multivalent binding. These multivalent tumor targeting peptides platforms have the potential to improve targeting in a variety of therapeutic applications.