Background: Heparin molecules possess immunomodulating properties, which are thought to complement their established antithrombotic activity. The purpose of this study was to evaluate whether the antiinflammatory properties of low molecular weight heparin (LMWH) can attenuate polymorphonuclear neutrophil accumulation and infarct size in a rat model of myocardial infarction. Methods: Myocardial infarction was induced by ligating the left main coronary artery. LMWH (fragmin 500 anti-FXa u/kg) or vehicle (saline) were administered subcutaneously thirty minutes prior to coronary artery occlusion. Significant anticoagulant activity was attained with LMWH for more than eight hours. Twenty-four hours later, neutrophil accumulation and infarct size were determined by measuring left ventricular free wall myeloperoxidase and residual creatine kinase activity, respectively. Results: As compared with rats administered vehicle, myeloperoxidase activity was insignificantly decreased in rats treated with LMWH (1.24 ± 0.28 u/g vs 1.66 ± 0.15 u/g, P=0.16). Infarct size was also not significantly different between the groups (62.48 ±3.5% and 50.67 ±7.2% of left ventricular free wall with vehicle and LMWH, respectively, P = 0.1). Conclusion: The authors conclude that LMWH does not significantly reduce myocardial neutrophil accumulation and infarct size twenty-four hours after myocardial infarction in the rat.