LyP-1 recognizes lymphatics and tumor cells in certain tumors, but not lymphatics in normal tissues. Screening on breast carcinoma xenografts shows positive to cyclic 9-amino-acid peptide, LyP-1. The LyP-1 also recognizes an osteosarcoma xenograft, and spontaneous prostate and breast cancers in transgenic mice. LyP-1 peptide is detected in tumor structures that are positive for several lymphatic endothelial markers and negative for blood vessel markers. LyP-1 accumulates in the nuclei of the putative lymphatic cells, and in the nuclei of tumor cells.