Objectives & Methods : In order to evaluate the in vivo synergic effect of Mawhangyounpye-tang which was a traditional poly-herbal formula has been used in the treatment of respiratory diseases in Korea, with quinolone antibiotic, ciprofloxacin (CPFX), the viable bacterial number and histopathological changes were monitored after experimental respiratory infection with Klebsiella peumoniae NCTC 9632. Results : The obtained results were as follows : 1. In CPFX group, 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 Mawhangyounpye-tang. 2. 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 group, and these were more dramatically decreased compared to that of single treatment with CPFX, respectively in concomitant treated groups with Mawhangyounpye-tang. 3. In CPFX group, 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 Mawhangyounpye-tang. Conclusions : It is considered that in vivo antibacterial activity of CPFX was dramatically increased by concomitant use of Mawhangyounpye-tang against Kebsiella pneumoniae NCTC 9632 infection of respiratory tract.