Repeated bleomycin administration in animals and humans produces significant lung fibrosis. The pathogenesis of this toxicity may be multifactorial, but it appears to be initiated through the production of radical oxygen species by an activated bleomycin-iron-oxygen ternary complex. Protection of lung tissue from bleomycin-induced toxicity may occur through both specific metabolic inactivation of bleomycin by the enzyme bleomycin hydrolase, as well as by such non-specific antioxidants as catalase and the glutathione system. The effect of chronic, systemic administration of bleomycin on the activities and levels of these enzymes and proteins in pulmonary tissue is unknown. C57BL/6 mice were injected subcutaneously with saline, non-fibrogenic (2 mg/kg) and fibrogenic (10 mg/kg) doses of bleomycin twice-weekly for 6 weeks. Animals were killed at 0, 1.5, 3, and 6 weeks after initiation of bleomycin treatment. Catalase activity was increased more than 50% at 3 weeks in the low-dose animals, and was decreased over 40% at 6 weeks in the high-dose animals. Total lung glutathione levels were unaffected in both groups, although glutathione reductase activity was increased significantly (over 2-fold) at 1.5 and 3 weeks in the high-dose animals. At 6 weeks glutathione reductase was increased 7- and 12-fold in low and high-dose animals respectively. Glutathione peroxidase activity also was elevated more than 2-fold above control values at 6 weeks in both sets of animals. There was no evidence of induction of bleomycin hydrolase activity at any time point. Rather, bleomycin hydrolase activity was decreased significantly to 30 and 40% of control values at 3 and 6 weeks, respectively, in mice receiving the fibrogenic doses of bleomycin. These results demonstrate that chronic, systemic administration of non-fibrogenic and fibrogenic doses of bleomycin produces changes in activity of lung antioxidant defense mechanisms. The early loss of lung bleomycin hydrolase activity may contribute to the pathogenesis of bleomycin-induced pulmonary toxicity following repeated drug exposure.