LyP-1 is a peptide selected from a phage-displayed peptide library that specifically binds to tumor and endothelial cells of tumor lymphatics in certain tumors. Fluorescein-conjugated LyP-1 and a related peptide, LyP-1b, strongly accumulated in primary MDA-MB-435 breast cancer xenografts and their metastases from i.v. peptide injections, allowing visualization of orthotopic tumors in intact mice. The LyP peptide accumulation coincided with hypoxic areas in tumors. LyP-1 induced cell death in cultured human breast carcinoma cells that bind and internalize the peptide. Melanoma cells that do not bind LyP-1 were unaffected. Systemic LyP-1 peptide treatment of mice with xenografted tumors induced with the breast cancer cells inhibited tumor growth. The treated tumors contained foci of apoptotic cells and were essentially devoid of lymphatics. These results reveal an unexpected antitumor effect by the LyP-1 peptide that seems to be dependent on a proapoptotic/cytotoxic activity of the peptide. As LyP-1 affects the poorly vascularized tumor compartment, it may complement treatments directed at tumor blood vessels.