Enterotoxigenic Escherichia coli (ETEC) of serotype O114:H21, which produced only heat-labile enterotoxin (LT), gave mannose-resistant hemagglutination (MRHA) with bovine erythrocytes. One strain, E20738A, was shown to possess fimbriae of approximately 7.5 nm diameter. On SDS-PAGE two possible fimbrial polypeptides of molecular masses 17.5 and 15.5 kDa were seen; the 17.5-kDa band was the most prominent. Loss of LT and MRHA together from strain E20738A was associated with loss of a 100-MDa plasmid. An absorbed anti-strain E20738A serum reacted specifically with the 17.5- and 15.5-kDa polypeptides and bound to the intact fimbriae. This antiserum reacted positively in an ELISA with LT-positive E. coli strains of serogroups O8, O15, O48, O114, and O146. The antiserum did not react with ETEC carrying known colonization factors. The term coli-surface-associated antigen (CS) 17 has been used to describe the fimbriae.