Since "small-dose and long-term" administration of erythromycin (EM) was shown to be efficacious in the treatment of chronic respiratory disease, the modulation of host defense responses by EM has attracted much attention. Although there is considerable controversy, it was recently demonstrated that EM activity reduces neutrophil function. In this study, we investigated the in vitro effects of the macrolides erythromycin (EM), a 14-membered ring, azithromycin (AZM), a 15-membered ring and rokitamycin (RKM), a 16-membered ring macrolide, on neutrophil function. The DCFH-DA method and cytochrome C method were used for assay of active oxygen generation and the Boyden-chamber method was used for assay of chemotaxis. EM and AZM, both of which have been shown to be clinically effective in the treatment of Diffuse panbronchiolitis (DPB), significantly suppressed active oxygen generation and chemotaxis of neutrophils at low concentrations equivalent to therapeutic doses (0.5 approximately 1.0 microgram/ml, p < 0.05), whereas the clinically ineffective RKM did not. The in vitro inhibitory effects of EM and AZM on active oxygen generation and chemotaxis of neutrophils demonstrated in the present study may be responsible for the therapeutic efficacy of these 14-membered and 15-membered ring macrolides in the treatment of DPB patients.