Galectin-3 is a member of the galectin family of beta-galactoside-binding animal lectins. Galectin-3 is overexpressed in a wide range of neoplasms and is associated with tumor growth and metastases. Given this fact, radiolabeled galectin-3-targeting molecules may be useful for the noninvasive imaging of tumors expressing galectin-3, as well as for targeted radionuclide therapy. In this study, the tumor cell-targeting and SPECT properties of a galectin-3-avid peptide identified from bacteriophage display were evaluated in human breast carcinoma cells and in human breast tumor-bearing mice. The galectin-3-avid peptide G3-C12 (ANTPCGPYTHDCPVKR) was synthesized with a Gly-Ser-Gly (GSG) linker at the amino terminus.