A survey of classical serotype enteropathogenic Escherichia coli has been made with respect to their plasmid profile and ability to adhere to HEp-2 cells. Thirty-one of the 32 strains examined contained a 50-70 Md plasmid, and many exhibited HEp-2 adherence. Strain E2348 (0127:H6), which causes diarrhea in volunteers and is HEp-2-adhesive, was chosen for further study. The large 55 Md plasmid in E2348, pMAR2, has been marked with a transposon coding for ampicillin resistance. E2348 that has been cured of pMAR2 loses the ability to adhere to HEp-2 cells, while HB101, a nonadherent E. coli K12, acquires HEp-2 adhesiveness after gaining this plasmid. Plasmid presence was also shown to correlate with in vivo adhesion to intestine, using the colostrum-deprived piglet model.