Particular strains of Escherichia coli that produce enterotoxins are an important cause of diarrheal disease in man and domestic animals. The ability of these enterotoxigenic E. coli (ETEC) strains to adhere to the intestinal epithelium is required as an initial step in establishing diarrheal disease. The bacterial cell surface structures that are responsible for adherence have been called adhesins or colonization factors (see also GAASTRA and DE GRAAF 1982, for a recent review). A special class of adhesins is formed by the proteinaceous filamentous surface appendages called fimbriae (DUGUID et al. 1955) or pili (BRINTON 1959). These fimbrial adhesins are host specific and include K88 (ORSKOV et al. 1964) and 987P (NAGY et al. 1976) on porcine strains; K99 (ORSKOV et al. 1975) and F41 (DE GRAAF and ROORDA 1982; MORRIS et al. 1982) on porcine, ovine and bovine strains; and CFA/I and CFA/II (EVANS et al. 1975; EVANS and EVANS 1978) associated with strains of human origin. Some characteristics of these fimbrial adhesins are shown in Table 1. It should be noted that this list is not complete and that the number of discovered fimbrial adhesins is rapidly increasing.