We previously showed increased expression of the ELR+, CXC chemokines in the lung after left pulmonary artery obstruction. These chemokines have been shown in other systems to bind their G-protein coupled receptor, CXCR₂, and promote systemic endothelial cell proliferation, migration, and capillary tube formation. In the present study, we blocked CXCR₂ in vivo using a neutralizing antibody and also studied mice that were homozygous null for CXCR₂. To estimate the extent of neovascularization in this model, we measured systemic blood flow to the left lung 14 days after left pulmonary artery ligation (LPAL). We found blood flow significantly reduced (67% decrease) with neutralizing antibody treatment compared to controls. However, blood flow was not altered in the CXCR₂ deficient mice compared to wild type controls after LPAL. To test for ligand availability, we measured MIP-2 protein in lung homogenates after LPAL, since this is the predominant CXC chemokine previously shown to be increased after LPAL (22). MIP-2 protein was 2-4 fold higher in the left lung relative to the right lung in all treatment groups 4 hrs after LPAL and this increase did not differ among groups. We speculate that the CXCR₂ deficient mice have compensatory mechanisms that mitigate their lack of gene expression and conclude that CXCR₂ contributes to chemokine-induced systemic angiogenesis after pulmonary artery obstruction.