Plasmid-encoded adherence factors have been shown to be important for the full expression of enteropathogenic Escherichia coli (EPEC) pathogenicity and for EPEC adhesion to cultured HEp-2 cells. EPEC strain E2348 (O127) shows localized HEp-2 cell adhesion and possesses a 60-megadalton plasmid, pMAR2. When E2348 is cured of pMAR2 it loses the ability to adhere to HEp-2 cells, while nonadherent E. coli K-12 strains P678-54 and HB101 acquire HEp-2 adhesiveness after they gain the plasmid. By electron microscopy, E2348 was seen to adhere to HEp-2 cells in a manner that closely resembled EPEC adhesion to intestinal mucosa; bacteria were intimately attached to projections of the apical HEp-2 cell membrane and caused localized destruction of microvilli. The plasmid-containing K-12 strains, on the other hand, did not show intimate attachment and there was no modification of cell surface architecture. It is concluded that plasmid pMAR2 codes for an adhesin, possibly fimbrial in nature, that promotes HEp-2 adhesion but that other chromosomally encoded factors are required for EPEC to achieve the characteristic mode of intimate cell attachment.