The levels of Escherichia coli K88 receptor were measured at various sites within the pig intestinal tract using an enzyme immunoassay. The amount of receptor in samples taken from K88-adhesive phenotype animals was found to vary widely along the length of the intestinal tract, but was usually highest in mucosal scrapings taken from the mid-small intestine. Receptor was evident in material collected near either end of the small intestine and was not apparent in material collected from the caecum or lower bowel. The ability of receptor-containing intestinal material to react with immobilized K88 adhesin was inhibited by exposure of the material to either trypsin or contents from the lower bowel, if the receptor-containing material was reacted with the immobilised K88 adhesin prior to exposure to trypsin or lower bowel contents, the bound material remained evident for 24 to 48 h. The possible implications of variable receptor activity in proteolytic environments in relation to pathogenesis and the determination of K88 phenotype in live pigs is discussed.