The following characteristics were found in 20 epidemic strains of enteropathogenic Escherichia coli (EPEC) 0127: H6 isolated from an outbreak of neonatal diarrhea: 1) Absent Vero toxin production; 2) No potential for invasiveness; 3) Possession of 1.5 and 60 Md plasmid identical restriction digest and outer membrane protein (OMP) patterns; 4) Ability of localized adherence to HEp-2, HeLa and FL cells; 5) Capability to cause diarrhea in rabbits with destruction of the ileal microvilli at the areas of bacterial adherence. After elimination of the 60 Md plasmid from EPEC 0127: H6 the 45 and 82 Kd OMPs of the parent strain were lost. These plasmid-cured strains became non-adherent to HEp-2, HeLa and FL cells, and unable to cause diarrhea in rabbits. These results suggest that the pathogenic mechanism of EPEC 0127: H6 induced diarrhea may be related to the genes on a 60 Md plasmid expressed as 45 and 82 Kd OMPs which cause localized adherence to epithelial cells and destruction of ileal microvilli. This damage leads to a marked reduction in absorptive surface area, resulting in diarrhea.