In order to evaluate the in vivo synergic effect of Eunkyo-san with quinolone antibiotics, rufloxacin (RUFX), the viable bacterial numbers and histopathological changes were monitored after experimental respiratory infection with Klebsiella peumoniae NCTC 9632. The obtained results were as follows : In RUFX group, the viable bacterial numbers were significantly decreased compared to those of control group and these were more dramatically decreased compared to those of single treatment with RUFX, 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 those of control group in RUFX group, and these were more dramatically decreased compared to those of single treatment with RUFX, respectively in concomitant treated groups with Eunkyo-san. In RUFX group, the LSA% (luminal surface of alveolar%) were significantly increased compared to those of control group and these were more dramatically decreased compared to those of single treatment with RUFX, respectively in concomitant treated groups with Eunkyo-san. According to these results, it is considered that in vivo antibacterial activity of RUFX group was dramatically increased by concomitant use of Eunkyo-san against K. pneumoniae NCTC 9632 infection of respiratory tract.