The role of colonisation factor antigen (C.F.A.) in diarrhœa was studied in volunteers who ingested either E.coli H-10407, which possessed C.F.A., or H-10407-P, which lacked C.F.A. In the first study the dose given was 10⁶ C.F.A.⁺ or C.F.A.^- E. coli; in the second study the dose was 10⁸ of the same organisms. Diarrhœa, with or without other symptoms, occurred only in those who ingested 10⁸ C.F.A.⁺ E. coli.Those volunteers taking the C.F.A.^- strain shed the test organism in the stool for only 2 or 3 days whereas those ingesting the C.F.A.⁺ strain shed the organism until the end of the study (7 days). Antibody responses in those ingesting the C.F.A.⁺ strain were: anti-C.F.A., 7 of 12 volunteers; antitoxin, 6 of 13; and anti-078 somatic antigen, 11 of 13. No volunteer ingesting the C.F.A.^- organism seroconverted to any of the antigens. The study showed the importance of C.F.A. in the pathogenesis of human disease due to toxigenic E. coli.