Limitation of reactive oxygen species mediated ischemia-reperfusion (I/R) injury of the lung by vascular immunotargeting of antioxidative enzymes might become a promising modality for extension of the viability of banked transplantation tissue. The preferential expression of angiotensin-converting-enzyme (ACE) in pulmonary capillaries makes it an ideal target for therapy directed toward the pulmonary endothelium. Conjugates of ACE monoclonal antibody 9B9 with catalase (9B9-CAT) have been evaluated in vivo for limitation of lung I/R injury in rats. Ischemia of the right lung was induced for 60min followed by 120min of reperfusion. Sham operated animals (sham; n=6) were compared with I/R untreated animals (I/R, n=6), I/R animals treated with biotinylated catalase (CAT, n=6) and I/R rats treated with the conjugates (9B9-CAT; n=6). 9B9-CAT accumulation in the pulmonary endothelium of injured lungs was elucidated immunohistochemically. Arterial oxygenation during reperfusion was significantly higher in 9B9-CAT (221±36mmHg) and sham (215±16mmHg; p<0.001 for both) compared to I/R (110±10mmHg) and CAT (114±30mmHg). Wet/dry weight ratio of I/R (6.78± 0.94) and CAT (6.54± 0.87) was significantly higher than of sham (4.85± 0.29; p<0.05), which did not differ from 9B9-CAT (5.58± 0.80). The lower degree of lung injury in 9B9-CAT treated animals compared to I/R rats was also shown by decreased serum levels of Endothelin-1 (sham: 18±9fmol/mg; I/R: 42±12fmol/mg; CAT: 36±11fmol/mg; 9B9-CAT: 26±9fmol/mg; p<0.01) and mRNA for iNO-synthase (iNOS/GAPDH ratio: sham: 0.15±0.06a.u.; I/R: 0.33±0.08a.u.; CAT: 0.26±0.05a.u.; 9B9-CAT: 0.14±0.04a.u.; p<0.001). These results validate immunotargeting by anti-ACE conjugates as a prospective strategy to augment antioxidative defenses of the pulmonary endothelium in vivo.