Tumor angiogenesis is a critical step for the growth and metastasis of solid tumors. Vascular endothelial growth factor (VEGF) is a specific and potent angiogenic factor and contributes to the development of solid tumors by promoting tumor angiogenesis. Therefore, it is a prime therapeutic target for the development of antagonists for treatment of cancer. We identified from peptide libraries arginine-rich hexapeptides that inhibit the interaction of VEGF(165) with VEGF receptor (IC(50) = 2-4 micrometer). They have no effect on binding of basic fibroblast growth factor to cellular receptor. The hexapeptides inhibit the proliferation of human umbilical vein endothelial cells induced by VEGF(165) without toxicity. The peptides bind to VEGF and inhibit binding of both VEGF(165) and VEGF(121), suggesting that the peptides interact with the main body of VEGF but not the heparin-binding domain that is absent in VEGF(121). The identified peptides block the angiogenesis induced by VEGF(165) in vivo in the chick chorioallantoic membrane and the rabbit cornea. Furthermore, one of the hexapeptides, RRKRRR, blocks the growth and metastasis of VEGF-secreting HM7 human colon carcinoma cells in nude mice. Based on our results, the arginine-rich hexapeptides may be effective for the treatment of various human tumors and other angiogenesis-dependent diseases that are related to the action of VEGF and could also serve as leads for development of more effective drugs.