The hst-1 transforming gene encodes a protein which belongs to the FGF family of growth factors. We showed previously that a human hst-1 protein produced in silkworm cells has in vitro mitogenic activity to vascular endothelial cells. Here we report effective synthesis of an unfused human hst-1 protein in E. coli and a potent in vivo angiogenic activity of this hst-1 protein by two in vivo assays for angiogenesis, chick chorioallantoic membrane assay and rat cornea assay. The NIH3T3 transformant transfected with the hst-1 gene appeared to develop a highly-vascularized tumor on nude mice. These data showed that the hst-1 protein has an angiogenic activity in vivo as well as in vitro.