F18(+) Escherichia coli can cause post-weaning diarrhoea and oedema disease in pigs. These diseases are responsible for substantial economic losses, but a vaccine is not available. A good knowledge of the characteristic of the fimbriae is useful for the development of a vaccine composed of the fimbrial virulence factor. F18 fimbriae are composed of the major subunit FedA and the minor subunits FedE and the adhesin FedF In the present study monoclonal antibodies (mAbs) against FedA and FedF were produced. In addition to their diagnostic value, these mAbs revealed a weaker interaction between FedA and FedF compared to the subunit-subunit interactions in other fimbriae, like type I and P pili. Further experiments are needed to investigate if this weak interaction could be one of the reasons for the slow colonisation of the small intestinal mucosa by F18(+) E. coli. (c) 2006 Elsevier B.V. All rights reserved.