Enterohemorrhagic Escherichia coli (EHEC) of serotype O157:H7 has two putative virulence factors: (i) a fimbrial adhesin, specified by a 60-megadalton (MDa) plasmid, and (ii) bacteriophage-specified cytotoxin(s), known as Shiga-like toxin (SLT) or verotoxin. The contribution of these factors to the pathogenesis of EHEC-induced disease in gnotobiotic piglets was examined. The bacterial strains included the following: two EHEC strains and their corresponding plasmid-cured derivatives; another EHEC isolate and its derivative which had spontaneously lost the ability to produce SLT; one E. coli K-12 transconjugatant containing a 60-MDa plasmid from an EHEC strain; two K-12 strains into which an SLT-producing phage had been transduced (one of these strains also carried a 60-MDa EHEC-derived plasmid); and the parent K-12 strain. Each strain was fed to four piglets, which were observed for diarrhea and examined for development of characteristic mucosal lesions 3 or 5 days after inoculation. All 24 piglets inoculated with the three EHEC strains and their respective derivatives (two plasmid cured and one SLT negative) showed the typical mucosal lesions of bacterial attachment: effacement of microvillous border and cell membrane dissolution culminating in destruction of surface and glandular epithelium in the cecum and colon. No such lesions were observed in 12 piglets inoculated with three strains of E. coli K-12, including the strain which carried both the 60-MDa plasmid and a phage which specified production of SLT. Moderate to severe diarrhea was observed in 16 piglets inoculated with two EHEC strains and their derivatives (one plasmid cured and one SLT negative). The third EHEC strain and its plasmid-cured derivative produced fewer typical mucosal lesions and no diarrhea. The reason for the reduced virulence of this strain was not clear. These results demonstrate that neither the 60-MDa plasmid nor the capacity to produce SLT is essential for expression of virulence by E. coli O157:H7 in gnotobiotic piglets.