The evaluation of tetracycline as a chemoprophylactic agent for cholera is described. Families of cholera patients were divided into 4 groups by strict rotation. The first group received multivitamin preparations and served as the control. The second received 1.0 g of tetracycline, divided into 4 doses, daily for 5 days, the third received 1.0 g of tetracycline in a single dose daily for 5 days, and the fourth received a single dose of 1.0 g of tetracycline. All families were visited daily for 10 days, a rectal swab being taken from each family member on each occasion. Tetracycline given daily for 5 days, either in divided doses or in a single daily dose, was effective in preventing subsequent infection. A single dose of tetracycline was less effective. The indications for chemoprophylaxis in cholera are discussed.