A clinical study was conducted on fleroxacin (FLRX) in 143 patients and carriers with infectious enteritis (shigellosis, Salmonella enteritis, Campylobacter enteritis, pathogenic Escherichia coli enteritis, Vibrio parahaemolyticus enteritis, cholera, multiple bacterial infections, pathogen-negative enteritis). Furthermore, its antibacterial activity against clinical isolates, fecal concentration and effect on fecal microflora were conducted. FLRX was administered orally in doses of 200 mg once a day (200 mg group) or 300 mg once a day (300 mg group) for 3 days to cholera, for 7 days to Salmonella enteritis and for 5 days to the other infectious enteritis. The clinical efficacy rates were 100% in both the 200 mg and 300 mg groups. The bacteriological efficacy rates were 100% against Shigella spp., Salmonella spp., pathogenic E. coli, V. parahaemolyticus and V. cholerae O1, and 63.6% against Campylobacter spp. in the 200 mg group. The rates of the 300 mg group were 93.3% against Shigella spp., and 100% against Campylobacter spp. and pathogenic E. coli. As adverse effects, skin rash was observed in 1 case each in both groups (1.1%, 2.1%). Abnormal laboratory findings consisted of 1 case of increased eosinophils and 1 case of elevated GOT and GPT levels in the 200 mg group (2.8%), and 1 case of elevated GPT in the 300 mg group (2.9%). The clinical usefulness rates were 92.9% and 93.3% in the 200 mg and 300 mg groups, respectively. Antibacterial activity was somewhat inferior to that fo ciprofloxacin and equal to or better than that of norfloxacin, demonstrating MIC90 values against Shigella spp., Salmonella spp., pathogenic E. coli, V. parahaemolyticus and Campylobacter spp. of 0.1, 0.2, 0.1, 0.2 and 0.78 micrograms/ml, respectively. Peak fecal concentrations of the drug were 49.0 micrograms/g and 274.4 micrograms/g in the 200 mg group, and 43.3 micrograms/g and below the detection limit (5.0 micrograms/g) in the 300 mg group. With respect to fecal microflora (4 cases), a decrease in Enterobacteriaceae was observed in 3 cases during dosing. But this change showed a tendency to recover after completion of dosing. No effects were observed on anaerobic bacteria.