Monoclonal antibodies against leukocyte adhesion molecules reduce the severity of reperfusion injury and, in some cases, prevent acute allograft rejection. These effects are assumed to be due to prevention of leukocyte- endothelial adhesion. Experiments were designed to study activation of polymorphonuclear leukocytes during acute rejection of canine single lung allografts and to test the ability of an anti-β2-integrin monoclonal antibody (R15.7/H4) to regulate leukocyte metabolism independent of leukocyte-endothelial adhesion. Peripheral blood polymorphonuclear leukocytes were obtained by Ficoll-Hypaque centrifugation from unoperated control dogs (n = 10), from dogs with a single lung autotransplant (n = 4), and from dogs after single lung allotransplantation during unmodified rejection (n = 12). Oxygen radicals were measured with luminal-enhanced chemiluminescence of unstimulated polymorphonuclear leukocytes, polymorphonuclear leukocytes stimulated by opsonized-zymosan, and polymorphonuclear leukocytes incubated with anti-β2-integrin β-chain (CD18) monoclonal antibodies before exposure to opsonized zymosan. Polymorphonuclear leukocyte viability was unaffected by the monoclonal antibodies. Polymorphonuclear leukocytes from unoperated and autotransplant dogs did not produce oxygen radicals unless stimulated by opsonized zymosan. Polymorphonuclear leukocytes from rejecting animals produced oxygen radicals in the absence of zymosan, and when stimulated, production of oxygen radicals was significantly greater than that of polymorphonuclear leukocytes from unoperated and autotransplant dogs. Production of oxygen radicals was inhibited significantly by the monoclonal antibodies in polymorphonuclear leukocytes from all three groups. This study shows that rejection activates polymorphonuclear leukocytes and increases the potential to produce activated oxygen species. In addition to inhibiting leukocyte adhesion to endothelial cells, anti-β2-integrin monoclonal antibodies appears to suppress production of oxygen radicals by polymorphonuclear leukocytes.