We describe the development and optimization of an in vivo angiogenesis assay utilizing gelfoam sponges impregnated with 0.4% agarose and different proangiogenic factors, such as basic fibroblast growth factor (bFGF), vascular epidermal growth factor (VEGF), tumor growth factor-alpha (TGF-alpha), and endothelial growth factor (EGF). The sponges are implanted into the subcutis of mice and harvested after different times. The gelfoam sponges are fixed, sectioned, and stained with fluorescent antibodies against CD31. The median number of CD31+ cells is determined in 10 different 0.159-mm2 fields. Proangiogenic molecules induced significant migration and proliferation of endothelial cells. To demonstrate the utility of this assay for evaluation of an antiangiogenic agent, mice were implanted with gelfoam sponges containing different proangiogenic factors and treated orally with water or PTK 787, a novel tyrosine kinase inhibitor with specific activity against the VEGF-R. PTK 787 significantly inhibited angiogenesis in sponges containing agarose + VEGF but not other proangiogenic molecules. The data show that the implanted gelfoam sponges provide a reliable quantitative assay to study in vivo angiogenesis.