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Background Recent studies have shown the LyP-1 peptide can home to either tumor lymphatics or the tumor cells and be intalized by targeted cells.This study aimed to investigate the possibility of using Na131I labeled LyP-1 peptide as an imaging agent or a therapeutic radiopharmaceutical in breast carcinoma and its metastasis.Methods The 10-mer cyclic peptide contained the LyP-1 sequence (YCGNKRTRGC) was synthesized by the solid phase method.Disulfide bonds between the cysteines maintain the cyclic structure.The LyP-1 peptide was labeled with Na131I using the chloramine-T method.The [131I] LyP-1 peptide and a [131I] control peptide were injected via tail vein into nude mice bearing MDA-MB-435 tumor xenografts.Biodistribution and imaging results in vivo were obtained.Results The labeling efficiencies of LyP-1 peptide reached 80％±5％ (n=5).The radiochemical purity was about 96％.The radiochemical purity of the labeled compound remains 92％ at 24 hours in human serum at 37℃.In the biodistribution studies,the [131I] LyP-1 peptide accumulated in the tumor to a higher level than in other organs.The [131I]LyP-1 peptide can successfully image the tumor in nude mice bearing MDA-MB-435 tumor xenografts.Conclusions The LyP-1 peptide could be effectively labeled with Na131I and the labeled compound is stable in human serum at 37℃ for 24 hours.The high specificity of [131I] LyP-1 peptide suggests it may be a promising new radiotracer for identifying tumors.