Neutrophils (polymorphonuclear leukocytes, PMNs) play a role in tissue injury after ischemia and reperfusion. We investigated the effects of a monoclonal antibody (MAb), PB1.3, directed against P-selectin in an acute model of myocardial ischemia-reperfusion injury. Dogs were subjected to 120 min of coronary arterial occlusion and 240 min of reperfusion. MAb PB1.3 (1 mg/kg), the nonblocking P-selectin antibody, MAb PNB1.6 (1 mg/kg), or saline was administered 5 min before reperfusion. Dogs treated with saline (n = 7), MAb PB1.3 (n = 7), and MAb PNB1.6 (n = 5) all experienced similar myocardial blood flows during ischemia, and treatment with MAb PB1.3 failed to preserve postischemic myocardial blood flow. Measurement of myocardial contractility failed to demonstrate any beneficial effects of MAb PB1.3 on postischemic myocardial contractility. However, myocardial necrosis (% of area at risk) was significantly reduced (P < 0.01) in dogs receiving MAb PB1.3 (20.8 +/- 4.8%) compared with dogs receiving either normal saline (41.7 +/- 4.5%) or MAb PNB1.6 (46.7 +/- 7.6%). Myocardial myeloperoxidase activity in the ischemic zone was 4.8 +/- 0.6 in the vehicle group and 3.7 +/- 0.5 in the MAb PNB1.6 group compared with 2.0 +/- 0.5 in MAb PB1.3-treated dogs (P < 0.01 vs. saline; P < 0.05 vs. PNB1.6). In summary, treatment with MAb PB1.3 failed to preserve postischemic myocardial blood flow or myocardial contractility. In contrast, P-selectin immunoneutralization reduced PMN accumulation and myocardial tissue injury in a canine model of coronary occlusion and reperfusion.