Pregnant swine (gilts) were vaccinated parenterally with a suspension of purified pili from the porcine enterotoxigenic Escherichia coli strain 987 (O9:K103:NM). Gilts injected with placebo served as controls. Suckling pigs born to gilts in both groups were challenged intragastrically with virulent strain 987. The percentage of deaths, incidence and duration of diarrhea, numbers of E. coli in the ilea and E. coli attachment to the villous epithelia were significantly less in suckling pigs of vaccinated gilts than in those of controls. These results are consistent with the hypothesis that pili of some enterotoxigenic E. coli facilitate adhesion to intestinal epithelia. Vaccination of dams with pili appears to be a means of immunizing against diarrhea caused by enterotoxigenic E. coli in suckling neonates. This work confirms the role of somatic pili as colonization and virulence factors and provides another example of safe and effective purified pilus vaccines.