Human thioredoxin, which was previously recognized as adult T-cell leukemia-derived factor, has many physiologic activities, one of which is a radical scavenger effect. Its ability to reduce reperfusion injury was assessed in vivo in a canine lung transplantation model. In 19 dogs, left lung allotransplantation was performed after 100 minutes of warm ischemia. The function of the transplanted lung was assessed after clamping of the contralateral pulmonary artery. In the human thioredoxin group (n = 6), human thioredoxin 30 mg/kg was given to the recipients during reperfusion. In the N-acetylcysteine group (n = 5), N-acetylcysteine 150 mg/kg, known as a radical scavenger, was given in the same manner. In both groups, arterial oxygen tension was significantly higher than in the control group (n = 8). In the human thioredoxin group, peak inspiratory pressure was significantly lower than in the control group. Macroscopic and microscopic examinations showed an almost normal appearance of the lung tissues in the human thioredoxin and N-acetylcysteine groups, in contrast to the abnormal findings in the control group. Thus it would appear that human thioredoxin has a protective effect on transplanted lungs, as does N-acetylcysteine, and that its action may be a radical scavenger effect.