Comparative fimbrial expression and adhesion studies were made on enterotoxigenic and verotoxigenic E. coli (ETEC and VTEC) strains isolated from cases of porcine postweaning diarrhoea or oedema disease. F107(F18ab) fimbriae--monitored by polyclonal and monoclonal antibodies and by electron microscopy--were poorly expressed on most VTEC strains. In contrast, 2134P(F18ac) fimbriae were more readily detected on most ETEC strains. The F18ac strains adhered in vivo to ligated intestinal loops in weaned pigs while the F18ab strains did not adhere or adhered weakly. Similarly, the F18ac strains adhered to isolated intestinal brush borders in weaned pigs but the F18ab strains (except for the F107 reference E. coli) did not adhere or adhered weakly in vitro. Neither the F18ab nor F18ac strains adhered to brush borders from newborn pigs. In vitro adhesion of F18ab and F18ac strains was mannose resistant and receptors for F18 seemed to differ from receptors for K88(F4). It is concluded that the antigenic variants of F18 fimbriae (F18ab and F18ac) are biologically distinct. F18ab fimbriae are expressed poorly both in vitro and in vivo and are frequently linked with the production of SLT-IIv and serogroup O139, while F18ac are more efficiently expressed in vitro and in vivo and most often are linked with enterotoxin (STa, STb) production, and serogroups O141, O157.