We have recently generated a monoclonal antibody (mAb), Phy-V002, which specifically labels activated vascular endothelial cells (EC) in zebrafish. Here, we show that this mAb labels activated EC in newly formed vessels in vivo without staining mature vessels or other tissues. Using this mAb, drug effects on in vivo EC migration and vessel formation were visually assessed by whole-mount immunochemical staining in the transparent embryo. In addition, we have developed a quantitative microplate-based ELISA that measures EC proliferation in vivo after drug treatment. We have validated the quantitative in vivo ELISA using several antiangiogenic small molecules with different mechanisms of action which were added directly to the fish water. Some of these drugs, including: 2-methoxyestradiol, flavopiridol, paclitaxel, and genistein, are currently in clinical trials. We also injected large molecule drugs, including 3TSR and TSR2+KRFK, recombinant human antiangiogenic peptides of thrombospondin-1, a natural protein. To demonstrate that proangiogenic effects can also be assessed in zebrafish, we assessed effects of penicillamine and simvastatin, two proangiogenic compounds shown to stimulate vessel formation in rodents. Using whole-mount immunochemical staining with Phy-V002, inhibition of EC migration and inhibition or stimulation of vessel formation were visually observed for each compound. Next, using the quantitative in vivo angiogenesis ELISA, we generated dose-response curves for each compound. Compared to conventional assays, advantages of using zebrafish to assess drug effects on angiogenesis include: (1) a short assay time; (2) easy animal maintenance; (3) use of small quantities of drug; (4) single dosing; (5) a quantitative assay format; and (6) use of statistically significant number of animals per test.