Vascular endothelial growth factor (VEGF) binding to its tyrosine kinase receptors (KDR/FLK1, Flt-1) induces angiogenesis. In search of the peptides blocking VEGF binding to its receptor KDR/FLK1 to inhibit tumor-angiogenesis and growth, we screened a phage display pep-tide library with KDR as target protein, and some candidate peptides were isolated. In this study, we cloned the DNA fragment coding the peptide K237 from the library, into a vector pQE42 to express fusion protein DHFR-K237 in E. coli M15. The affection of fusion protein DHFR-K237 on endothelial cell proliferation and angiogenesis was investigated. In vitro, DHFR-K237 could completely block VEGF binding to KDR and significantly inhibit the VEGF-medi-ated proliferation of the human vascular endothelial cells. In vivo, DHFR-K237 inhibited angiogenesis in chick embryo chorioa- llantoric membrane and tumor growth in nude mice. These results suggest that K237 is an effective antagonist of VEGF binding to KDR, and could be a potential agent for Vascular endothelial growth factor (VEGF) binding to its tyrosine kinase receptors (KDR / FLK1, Flt-1) induces angiogenesis. In search of the peptides blocking VEGF binding to its receptor KDR / FLK1 to inhibit tumor- angiogenesis and growth, we screened a phage display pep-tide library with KDR as target protein, and some candidate peptides were isolated. In this study, we cloned the DNA fragment coding the peptide K237 from the library, into a vector pQE42 to express fusion protein DHFR-K237 in. In vitro, DHFR-K237 could completely block VEGF binding to KDR and significantly inhibit the VEGF-mediate-ated proliferation of the human vascular endothelial cells. In vivo, DHFR-K237 inhibited angiogenesis in chick embryo chorioa-llantoric membrane and tumor growth in nude mice. These results suggest that K237 is an effective antagonist of VEGF binding to KDR, and could be ap otential agent for