Miloxacin, a novel chemotherapeutic agent, has been reported to have very potent antibacterial activity to E. coli, Klebsiella, Proteus and other Gram-negative rods and be excreted into bile at high levels. Therefore, the effectiveness of miloxacin against biliary tract infections and its excretion into bile in patients were investigated. The results were as follows: i) Comparative studies on the excretion of miloxacin, nalidixic acid (NA) and cephalexin (CEX) into bile of patients operated in gallbladder were performed by a crossover method. The maximum concentration of miloxacin after oral administration of 500 mg ranged from 18.2 to 24.0 micrograms/ml and were much higher than those of NA (5.3 micrograms/ml) and CEX (3.4 micrograms/ml). The mean recovery of miloxacin within 6 hours was 0.256% and much greater than those of NA (0.075%) and CEX (0.027%). ii) Into bile, intact miloxacin and its metabolites (the glucuronides of miloxacin and M-2) were excreted but M-1 was scarcely excreted. Regression analysis of the concentrations by the bioassay compared with those intact miloxacin by the HPLC method gave a good correlation with r = 0.96. iii) The clinical effectiveness against the biliary tract infections was investigated in 29 cases. Miloxacin showed the excellent effect in 8 cases and the good effect in 17 cases, giving an effective ratio of 86.2%. Especially, all of the cases dosed with miloxacin over 34 mg/kg in a day showed excellent or good results. iv) The rates of clinical effectiveness were 85.7, 80.0% and 71.4% in E. coli, Klebsiella and Proteus infections, respectively. The antibacterial activity of miloxacin against 27 strains isolated in this test was more potent than those of NA and CEX. v) During the clinical tests, none of any side effect with miloxacin was observed in both of subjective and objective symptoms. These results show that miloxacin is expected to be fully applicable on the therapy of biliary tract infections.