Vascular endothelial growth factor (VEGF) mediates tumor angiogenesis, growth, and metastasis. Murine models of metastatic tumors in which green fluorescent protein (GFP) expression is driven by the VEGF promoter can be imaged both intravitally and externally and thus offer many possibilities for real-time studies of tumor angiogenesis, metastasis, and treatment in vivo. In our defined-flora animal facility, an 11-month-old female transgenic mouse with a C3H background (VEGF(P)-GFP/C3H) developed a spontaneous tumor that expressed GFP under the control of VEGF. Necropsy and histopathologic examination revealed an osteosarcoma with lung metastases. Fresh tumor fragments were transplanted successfully into other VEGF(P)-GFP/C3H transgenic mice. During the first five generations, the tumor "take rate" was 100% (25 of 25 animals), with a latent period of 8 days and an average tumor volume of 1500 mm3 at 36 days. Transplanted tumors have maintained their original histopathologic characteristics and metastatic behavior. In addition, the tumor grows in wild-type C3H mice with an 83% take rate (10 of 12 animals) and as monolayer cells in vitro. GFP was expressed strongly in tumor tissue, lung metastatic foci, and cultured tumor cells. Real-time growth of tumors grown in dorsal skin chambers in C3H mice could be visualized using GFP fluorescence. In addition, GFP fluorescence of metastatic lesions in lungs of C3H mice was clearly visible by multiphoton laser scanning microscopy. This in vitro and in vivo transplantable and metastatic osteosarcoma (Os-P0107) is an attractive model for further study of tumor pathophysiology and treatment efficiency affecting VEGF expression.