Coronary collateral development relies on the coordinated secretion of growth factors. However, alone they are insufficient for permanent collateral growth. We utilized proteomics to identify other important proteins in the extracellular environment that could facilitate collateralization. Chronically instrumented dogs developed coronary collaterals by the repetitive occlusion method. Subendocardial blood flow (day 1=0.19 ±0.04, day 7= 0.27 ±0.06, day 14= 0.48 ±0.10, day 21= 0.81 ± 0.11 ml/min/g) and subepicardial blood flow (day 1=0.14 ±0.01, day 7=0.36 ±0.06, day 14=0.51 ±0.07, day 21= 0.71±0.08 ml/min/g) increased in repetitive occlusion animals. Sham animals exhibited no changes in blood flow. Myocardial interstitial fluid (MIF) from both groups underwent 2D electrophoresis with MALDI-tof identification. The acute phase protein haptoglobin was identified in the repetitive occlusion group. ELISA of MIF showed haptoglobin to be elevated at all time points of collateral development when compared to sham, with maximal production on day 7. Purified haptoglobin dose dependently stimulated endothelial cells (EC) to form tubes, and vascular smooth muscle (SMC) cells to migrate. Purified haptoglobin did not stimulate either cell type to proliferate. The relative contribution of haptoglobin to the chemotactic properties of MIF was tested using a neutralizing antibody. Neutralized MIF could not stimulate SMC to migrate at any time point of collateral development. EC tube formation was inhibited after the midpoint of collateralization. Therefore, the acute phase protein haptoglobin plays a critical role during coronary collateralization.