pMAR2, a 60-megadalton plasmid encoding localized HEp-2 adherence in enteropathogenic Escherichia coli, was mapped with BamHI, HindIII, and SalI. Deletion and insertion mutants were constructed and used to define a potential DNA probe. Preliminary results indicate that this probe is sensitive and specific for the genes encoding the enteropathogenic E. coli adherence factor.