In vitro effects of macrolide clarithromycin (CAM) on influenza A virus-infected cells were examined using plaque reduction assay by treating cells either before or after viral adsorption. The significant inhibitory effect on influenza virus infection was detected only when the cells were treated with CAM after viral adsorption. The predominant inhibitory effect was observed during 4-7th hour after viral adsorption using viral production assay. CAM did not exhibit inhibitory effects on influenza virus hemagglutination, membrane fusion and viral sialidase activities. These findings indicate that CAM acts on a middle to late stage of the viral replication cycle resulting in inhibition of progeny virus production from the infected cells.