In order to evaluate the in vivo synergic effect of Eunkyo-san which was a traditional poly-herbal formula has been used in the treatment of respiratory diseases in Korea, with quinolone antibiotics, ciprofloxacin (CPFX), the viable bacterial number and histopathological changes were monitored after experimental respiratory infection with S. pneumoniae ATCC 6303. The obtained results were as follows: In CPFX groups, the viable bacterial numbers were significantly decreased compared to that of control group and these were more dramatically decreased compared to that of single treatment with CPFX, respectively in concomitant treated groups with Eunkyo-san. In control group, severe infiltration of inflammatory cells, hemorrhage and hypertrophy of alveolar linings were demonstrated at microscopical levels. However, these abnormal histopathological changes were significantly decreased compared to that of control group in CPFX groups, and these were more dramatically decreased compared to that of single treatment with CPFX, respectively in concomitant treated groups with Eunkyo-san. In CPFX groups, the LSA (luminal surface of alveoli $\%$) were significantly increased compared to that of control group and these were more dramatically decreased compared to that of single treatment with CPFX, respectively in concomitant treated groups with Eunkyo-san. According to these results, it is considered as the in vivo antibacterial activity of CPFX was dramatically increased by concomitant use of Eunkyo-san against S. pneumoniae ATCC 6303 infection of respiratory tract.