Doxycycline was compared with tetracycline in the treatment of cholera. Four types of treatment were compared: Group A was given 200 mg of doxycycline on admission and 100 mg on the second day; Group B was given 200 mg of doxycycline on admission only; Group C was given 300 mg of doxycycline on admission only; and Group D received 500 mg of tetracycline every 6 h for 48 h. Tetracycline showed a slight advantage in respect of duration of diarrhoea and vibrio excretion compared with doxycycline given as a single dose of 300 mg, but fluid intake and output were about the same in these two groups. The other two doxycycline treatment schedules did not compare well with tetracycline treatment.