Enterotoxigenic Escherichia coli (ETEC) is the most commonly isolated pathogen responsible for travelers' diarrhea and the cause of up to 650 million cases of pediatric diarrhea per year in the developing world. As a safe alternative to the prophylactic use of antibiotics, a hyperimmune bovine milk antibody product with specific activity against purified colonization factor antigens (CFAs) was developed and evaluated in a human challenge study. Twenty-five healthy adult volunteers were challenged orally with 10⁹ cfu of a virulent CFA/I-bearing ETEC. In the randomized doubleblind trial, 7 of 10 volunteers receiving a lactose-free placebo developed clinical diarrhea after challenge, compared with only 1 of 15 cases in volunteers receiving active product (Fisher's exact test, P < .0017). It is concluded that antibodies against CFAs alone are sufficient for protection and that prophylaxis with milk-derived immunoglobulin is a feasible alternative to existing drug interventions.