A DNA probe to detect genes conferring localized adherence of enteropathogenic Escherichia coli (EPEC) to Hep-2 cells was evaluated by using E. coliisolates from the stools of Peruvian infants with and without diarrhea. The probe was both sensitive and specific and revealed that Hep-2 adherence (because of the EPEC adherence factor [EAF]) was more frequent in some O serogroups of EPEC than in others. Those serogroups in which EAF is almost always found have been designated class I EPEC; serogroups in which EAF is rarely found have been designated class II. Both class I (EAF-positive) and class II EPEC are associated with diarrheal disease. © 1985 by The University of Chicago.