Objectives & methods; In order to evaluate the in vivo synergic effect of Mawhangyounpye-tang, a traditional poly-herbal formula used in the treatment of respiratory diseases in Korea, with the quinolone antibiotic ciprofloxacin (CPFX), the viable bacterial number and histopathological changes were monitored after experimental respiratory infection with Streptococcus Pneumoniae ATCC 6303. Results: 1. In CPFX groups, the viable bacterial numbers were significantly decreased compared to that of the control group, and were even more dramatically decreased in concomitant group treated with Mawhangyounpye-tang. 2. In the control group, severe infiltration of inflammatory cells, hemorrhage and hypertrophy of alveolar linings were demonstrated at microscopic levels. However, these abnormal histopathological changes were significantly decreased compared. to that of the control group in CPFX groups, and were even more dramatically decreased in concomitant groups treated with Mawhangyounpye-tang. 3. In CPFX groups, the LSA (Iuminal surface of alveoli $\%$) were significantly increased compared to that of the control group, and more dramatically in concomitant groups treated with Mawhangyounpye-tang. Conclusions: According to these results, it is considered that the in vivo antibacterial activity of CPFX against Streptococcus Pneumoniae ATCC 6303 infection of respiratory tract was dramatically increased by concomitant use of Mawhangyounpye-tang.