During the first year of life a group of babies was prospectively observed for diarrhea and for fecal carriage of heat-labile toxigenic bacteria, with or without colonization factor, and rotavirus. Approximately half of the babies were breast-fed for the first six months of life. There was no difference between groups (breast-fed vs non-breast-fed) in number of babies who had diarrhea during any two-month period. Nor was there any difference between groups in the number of babies who had diarrhea while carrying toxigenic bacteria, with or without colonization factor. Secretory antibody to toxin was found in 37% of colostrum and milk samples. There was a small but insignificant difference in the number of babies who had diarrhea when they carried toxigenic bacteria depending on the presence of antibody in the breast milk they received.