The purpose of the study reported here was to compare the bactericidal effectiveness of tetracycline and co-trimoxazole (a combination of sulfamethoxazole and trimethoprim) in treating cholera. The study, an open-ended random trial using adult patients with cholera cases confirmed by stool culture, was carried out in March 1993 at the Cholera Treatment Unit (CTU) of the Hospital de Apoyo Departmental María Auxiliadora in Lima, Peru. A total of 107 subjects were divided into two groups (A and B). The 50 in Group A received 500 mg of tetracycline orally every 6 hours for 3 days; the 57 in Group B received co-trimoxazole (160 mg of trimethoprim and 800 mg of sulfamethoxazole) orally every 12 hours for 3 days. The two groups were comparable in terms of age, sex, duration of symptoms prior to hospital admission, time at which antibiotic treatment was initiated, and clinical evolution. Control stool cultures of specimens obtained after treatment showed Vibrio cholerae O-1 present in 2% of the Group A and 12.3% of the Group B patients, and also showed V. cholerae non-O-1 present in 2% of the Group A patients and 3.5% of the Group B patients. Overall, it was concluded that both therapeutic treatment regimens were effective and that the strains of V. cholerae observed in the southern sector of the city of Lima were still susceptible to both antibiotics.