Three enteropathogenic Escherichia coli (E.P.E.C.) strains (O127:K63:H6, O128:K67:H2, and O142:K86:H6) isolated from outbreaks of infantile diarrhoea and one strain from the "normal" colonic flora (E. coli HS) of a healthy adult were fed in doses of 10(6), 10(8), and 10(10) organisms in NaHCO3 to adult volunteers. The strains, which had been stored for 7--9 years, gave negative results in sensitive tests for heat-labile (L.T.) enterotoxin (Y-1 adrenal-cell test), heat-stable (S.T.) enterotoxin (infant mouse assay), invasiveness (guineapig eye test), and gross fluid accumulation (infant rabbit assay). Two strains (O142 and O127) caused diarrhoea. L.T. or S.T. enterotoxins were not found in E. coli stool isolates from individuals with diarrhoea and no one had a rise in L.T. antitoxin titre; the findings suggest that L.T. and S.T. enterotoxins were not involved in pathogenesis of the diarrhoea. Non-invasive E.P.E.C. strains probably induce diarrhoea by a mechanism (presumably an enterotoxin) distinct from L.T. or S.T. enterotoxins.