Three enteropathogenic Escherichia coli(E.P.E.C.) strains (0127:K63:H6, 0128:K67:H2, and 0142:K86:H6) isolated from outbreaks of infantile diarrhœa and one strain from the "normal" colonic flora (E. coli HS) of a healthy adult were fed in doses of 10⁶, 10⁸, and 10¹⁰ 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 adrenalcell test), heat-stable (S.T.) enterotoxin (infant mouse assay), invasiveness (guineapig eye test), and gross fluid accumulation (infant rabbit assay). Two strains (0142 and 0127) caused diarrhœa. L.T. or S.T. enterotoxins were not found in E. coli stool isolates from individuals with diarrhœa 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 diarrhœa. Non-invasive E.P.E.C. strains probably induce diarrhœa by a mechanism (presumably an enterotoxin) distinct from L.T. or S.T. enterotoxins.