HER2/neu and MUC1-based synthetic peptides were prepared and evaluated in an effort to develop peptide-based radiopharmaceuticals derived from tumor-associated-antigens for the detection of breast cancer. The receptors for HER2/neu and MUC1 are overexpressed in various human cancers, such as breast and ovarian cancer. The relatively low expression of these antigens on normal tissues makes them attractive targets for tumor imaging. In addition, antitumor-antibody-derived peptides based on the Glu-Pro-Pro-Thr (EPPT) sequence were prepared for the detection of breast cancer. It has been shown that the EPPT peptide has high affinity for the MUC1-derived peptide. The peptides were prepared by solid-phase synthesis and radiolabeled efficiently with (99m)Tc via ligand exchange. They exhibited good stability in vitro in human plasma and against cysteine and histidine challenge. The peptides displayed high affinities (in nanomolar range) for MCF-7, MDA-MB-231 and T47-D breast cancer cell lines in vitro. Additionally, they exhibited a rapid internalization into tumor cells. In vivo biodistribution in mice showed rapid and efficient blood clearance and excretion mainly through the renal-urinary route, with some elimination via the hepatobiliary pathway. However, the extent of urinary excretion was found to be variable for radiopeptides, with the highest being observed for antitumor-antibody-derived peptide. The peptides showed moderate tumor uptake (up to 2.2+/-0.98% ID/g) in nude mice carrying breast tumor xenografts. The uptake in the tumor was always higher than in the blood and muscle. A fast clearance from the blood and low accumulation (<6% ID/g) by the major organs was obtained in nude mice resulting in favorable tumor/blood and tumor/muscle ratios as early as 1 h after injection. The combination of favorable in vitro and in vivo characteristics makes this new and interesting class of peptides potential candidates for the diagnosis of breast cancer in vivo.