This study was performed to determine whether the F1 (type 1) pili of a porcine strain of enterotoxigenic Escherichia coli are protective antigens and whether they are produced in the pig small intestine during disease caused by an enterotoxigenic E. coli. Reciprocal cross-absorption experiments with antisera prepared against F1 pili purified from enterotoxigenic E. coli 431 (O101:K30,99:H-:F1) and P14 (O149:K91,88ac:8H+:F1) demonstrated that the F1 antigens of the two strains were closely related or identical. Pregnant swine vaccinated with a vaccine prepared from strain P14 (F1+) responded with a significant increase in antibody against F1 in their serum and colostrum. However, the vaccinated dams did not significantly protect their suckling pigs against fatal challenge with strain 431. There was no evidence of F1 pilus production in the strain 431-infected pigs, as determined by immunofluorescent staining of ileal sections, direct electron microscopic examination of bacteria from ilea, and titration of serum agglutinins in convalescent pigs. It was concluded that strain 431 did not produce F1 in the small intestine during disease and that F1 was not a protective antigen in this system.