Several methods for the in vivo study of angiogenesis are available, and each angiogenic assay presents distinct advantages and disadvantages. In this study, we present a new method for the quantitation of angiogenesis and antiangiogenesis in the chick embryo chorioallantoic membrane (CAM), based on the implantation of gelatin sponges on the top of growing CAM, on day 8 of incubation. After implantation, the sponges were treated with a stimulator (recombinant human basic fibroblast growth factor, FGF2) or an inhibitor (a rabbit polyclonal anti-FGF2 antibody) of blood vessel formation. Blood vessels growing vertically into the sponge and at the boundary between sponge and surrounding CAM mesenchyme were counted by a morphometric method on day 12. In addition, to assess whether the gelatin sponge is an appropriate vehicle to deliver cultured cells and evaluate their angiogenic potential, mouse aortic endothelial cells were cotransfected with human FGF2 and the Escherichia coli beta-galactosidase (beta-GAL) reporter gene. Stable transfectants were absorbed by the sponge, and evaluation of the angiogenic response was paralleled by beta-GAL staining to visualize implanted cells. This technique may facilitate the discovery and development of agonists or antagonists of angiogenesis.