Lomefloxacin (NY-198), a new antimicrobial quinolone, was examined for its antimicrobial activities against clinical isolates and clinical efficacies to biliary tract infections. The following results were obtained. 1. The MICs of NY-198 against Escherichia coli (20 strains) and Klebsiella pneumoniae (20 strains) were good and similar to those of ofloxacin (OFLX) or norfloxacin (NFLX). The MICs of NY-198 against Pseudomonas aeruginosa (20 strains) were inferior by 1 dilution factor to OFLX or NFLX, and against Enterococcus faecalis (10 strains), they were similar to NFLX and slightly inferior to OFLX. 2. NY-198 was administered to 8 patients with biliary tract infections (acute cholecystitis 7 cases, chronic cholangitis 1 case). The results were good in 7 and unevaluable in 1 case because the duration of the therapy was too short. 3. As for side effects, mild urticaria was observed in 1 case and epigastralgia with nausea in another. As for abnormal laboratory test values slight elevations of GOT and GPT were recognized in 1 case. 4. In conclusion, we consider NY-198 is a useful oral drug for the treatment of biliary tract infections.