The role of colonisation factor antigen (C.F.A.) in diarrhoea 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 10(6) C.F.A.+ or C.F.A.- E. coli; in the second study the dose was 10(8) of the same organisms. Diarrhoea, with or without other symptoms, occurred only in those who ingested 10(8) 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-O78 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.