Four phenotypes of pigs distinguished by the variant(s) of K88 fimbrial adhesin (K88ab, K88ac, K88ad) that bind to their intestinal epithelial cells (I-none of the variants, II-K88ad, III-K88ab and K88ac, and IV-all three variants) have been identified. We hypothesize that the differences between the phenotypes are defined by the presence or absence of K88 adhesin receptors. We propose a three-receptor model to account for the observed phenotypes: 1) Receptor bed which binds all three variants and is found in phenotype IV, 2) Receptor be which binds K88ab and K88ac and is found in phenotype III and IV, and 3) Receptor d which binds K88ad and is found in phenotype II. We have identified the be receptor activity as a pair of mucin-type sialoglycoproteins (210 and 240 kDa). Although neither the bcd nor d receptor has been identified biochemically, their presence has been established using both blocking and receptor localization studies. Blocking studies using phenotype IV brush borders demonstrated that K88ab and K88ac fimbriae block the binding of E. coli expressing any of the K88 variants, but K88ad fimbriae block only K88ad E. coli binding. These results indicate that two receptors (bcd and bc) exist in the phenotype IV animals. Receptor localization studies on intestinal sections from phenotype IV animals showed that K88ab and K88ac adhesin binding is continuous from the crypt to the tip of the villus. The binding of the K88ad adhesin binding is multifocal in phenotype IV pigs, but continuous from crypt to tip of the villus in sections of phenotype II pigs. These studies verify the presence of two receptors (bcd and bc) in phenotype IV animals, and indicate that the K88ad receptor in phenotype IV animals (bcd) is different than in phenotype II animals (d).