In a prospective controlled study we evaluated the effect of early norfloxacin treatment on the duration of salmonella carriage after acute salmonellosis. The study was carried out during an outbreak of Salmonella typhimurium infection at a military base. 23 patients received norfloxacin 400 mg twice daily for 7 days while 29 patients served as untreated controls. A patient was considered to have ceased being a carrier on the date of the first of 3 negative consecutive cultures. Four weeks after diagnosis 30% of the treated patients and 31% in the control group were still carriers. The corresponding figures after 8 and 12 weeks were 17 and 3% and 4 and 0%, respectively. Thus, one week of norfloxacin treatment instituted at an early stage of salmonellosis did not shorten the duration of carriage.