Of 14 strains of Escherichia coli O157:H7 isolated from patients with hemorrhagic colitis or hemolytic uremic syndrome that were examined for fimbriae, the presence of plasmids, and the ability to adhere to intestinal cells, 13 possessed a 60-megadalton plasmid and were fimbriated as assessed by electron microscopy. These strains adhered to Henle 407 intestinal cells but not to HEp-2 cells or erythrocytes. Three strains were cured of the plasmid and thereafter failed to express fimbriae and lost the ability to adhere to intestinal cells. Conversely, E. coli K-12 transformed with the 60-megadalton plasmid from each of the three strains produced fimbriae and was able to adhere to intestinal cells. A single fimbrial subunit of 16 kilodaltons was observed when purified fimbriae from the transformants and from the 60-megadalton plasmid-containing E. coli O157:H7 strains were disaggregated and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antisera raised against one preparation of the purified fimbriae reacted strongly with 12 of 14 O157:H7 isolates in an agglutination assay and with purified fimbrial preparations from five E. coli O157:H7 strains in an enzyme-linked immunosorbent assay.