We performed a prospective study to examine whether the IgA antibodies against cholera that are present in breast milk protect breast-fed infants and children against colonization with Vibrio cholerae 01 and disease. Among families of patients with cholera, we collected breast milk from mothers who had not had diarrhea in the previous week and monitored them and their breast-fed children for cholera colonization and diarrhea for 10 days. Breast milk was assayed for IgA antibodies to cholera toxin and lipopolysaccharide. Ninety-three mother--child pairs were studied; 30 infants became colonized with V. cholerae 01 and disease developed in 19. There were no differences between the antibody levels in milk fed to children who became colonized and in milk fed to children who did not. However, among the children who became colonized, those who had diarrhea drank breast milk containing significantly lower levels of both kinds of cholera antibodies than were present in the milk consumed by children who had no symptoms. We conclude that breast-milk antibodies against cholera do not appear to protect children from colonization with V. cholerae 01 but do protect against disease in those who are colonized.