BACKGROUND Diffuse panbronchiolitis (DPB) is characterized by chronic neutrophil-mediated inflammation of the airway mediated by oxygen radical production. DPB can be controlled with low-dose and long-term erythromycin therapy based on its anti-inflammatory effect. OBJECTIVE In this study, the antioxidant levels were analyzed as an anti-inflammatory effect of erythromycin in the patients. METHODS We investigated the activity and protein level of an antioxidant enzyme, Cu, Zn-superoxide dismutase (SOD) in alveolar macrophages (AMs) of patients with DPB before and after erythromycin therapy. AMs were obtained from bronchoalveolar lavage fluid. RESULTS There was no significant difference in the activity of Cu, Zn-SOD between normal subjects and untreated patients. Erythromycin therapy (600 mg/day) significantly increased the activity of the enzyme relative to that before therapy and normal subjects [18.2 units/10(6) cells (9.2-26.2) vs. 4.4 units/10(6) cells (1.1-9.3), p < 0.01 and 10.4 units/10(6) cells (2.4-20.6), p < 0.05, respectively]. Furthermore, the protein level of Cu, Zn-SOD in AMs in treated patients was significantly higher than in the other two groups [69.4 ng/10(6) cells (34.2-147.1) vs. 20.1 ng/ 10(6) cells (16.9-39.8) for untreated patients, p < 0.01 and 43.2 ng/10(6) cells (32.6-68.2) for normal subjects, p < 0.01], but the levels in the latter groups were not different. CONCLUSIONS Our results suggest that one of the anti-inflammatory effects of erythromycin in DPB may be, in part, mediated by enhancement of antioxidant activity in AMs.