A quantitative in vivo angiogenesis model employing collagen onplants placed on the chick embryo chorioallantoic membrane (CAM) has been used in this study to assess the spatial and temporal associations between neutrophil-like inflammatory cells, namely chicken heterophils, and the development of new blood vessels. Previously we have demonstrated that monocytes/macrophages infiltrating the onplants were associated with extracellular matrix remodeling and angiogenesis, in particular by delivering MMP-13 collagenase. By introducing chicken gelatinase B (chMMP-9) as a specific marker for heterophils, we now show that the onset and extent of angiogenesis induced by purified growth factors or by human HT-1080 fibrosarcoma cells correlated with the initial influx of chMMP-9-positive heterophils. This early heterophil arrival was followed by the infiltration of monocytes/macrophages and appeared to sustain further blood vessel formation. The disruption of inflammatory cell influx by 2 mechanistically distinct anti-inflammatory drugs, cortisone and ibuprofen, significantly inhibited angiogenesis, indicating a functional involvement of these inflammatory cells in new blood vessel development. A direct addition of isolated heterophils or purified chMMP-9 into the HT-1080 onplants engrafted into cortisone- or ibuprofen-treated embryos reversed the antiangiogenic effects of the drugs. The exogenously added heterophils induced in vivo a further infiltration of endogenous heterophils and monocytes and dramatically rescued the impaired angiogenesis, highlighting the importance of early inflammatory leukocytes in tumor-induced angiogenesis. Moreover, purified heterophils incorporated into onplants lacking growth factors or tumor cells induced angiogenesis in nontreated embryos, further indicating a direct proangiogenic role for neutrophil-like leukocytes.