We generated germline transgenic silkworms bearing a fibroin light chain (FL) promoter-driven FL gene whose 3'-end was flanked with human basic fibroblast growth factor (bFGF) gene, FL/bFGF gene. The cocoons from transgenic worms were trypsinized to remove sericin layers, and treated with solution containing CaCl(2), ethanol, and water at a molar ratio of 1:2:8 (CaCl(2)/ethanol/water) to solubilize fibroin layers. Western blot analysis showed that the recombinant protein, r(FL/bFGF), was solubilized with CaCl(2)/ethanol/water, but not with trypsin, indicating that r(FL/bFGF) was in fibroin layers. Thus, it was concluded that the worms spun cocoons whose fibroin layers were composed of the inherent gene-derived natural fibroin (nF) and r(FL/bFGF). The mixture of nF and r(FL/bFGF) was dubbed r(FL/bFGF)nF. The solubilized r(FL/bFGF)nF was refolded using the glutathione redox system. Human umbilical vein endothelial cells (HUVECs) grew in the refolded r(FL/bFGF)nF-containing culture media, showing that bFGF in r(FL/bFGF) was biologically active. r(FL/bFGF)nF immobilized on a culture dish also supported the growth of HUVECs in bFGF-free media, suggesting the usefulness of r(FL/bFGF)nF as a new biomaterial for tissue engineering. The currently developed transgenic silkworms will be suitable for mass production of fibroins bearing a variety of biological activities.