The in vitro and in vivo antimicrobial potencies of KY-109, a pro-drug of KY-087, were compared with those of amoxicillin, cephalexin (CEX), and cefaclor (CCL). The following results were obtained. KY-087, which is the active form of KY-109, had broad antimicrobial spectrum against Gram-positive and Gram-negative organisms, but showed low antimicrobial activity against Enterobacter sp., Serratia, and Pseudomonas sp. The antimicrobial activities of KY-087 against clinically isolated Gram-positive organisms were superior to those of CEX and CCL. The antimicrobial activities of KY-087 against Gram-negative organisms, such as Enterobacter sp., Serratia, and Pseudomonas sp., were less active. KY-087 showed dose-related bactericidal activity against Staphylococcus aureus and Escherichia coli. The therapeutic efficacy of KY-109 against experimental intraperitoneal infections caused by Gram-positive and Gram-negative organisms in mice was comparable to that of CEX but inferior to that of CCL. In experimental granuloma pouch models in rats and kidney infection in rabbits, therapeutic efficacy of KY-109 was either comparable or superior to that of CEX and CCL.