The ability of Escherichia coli K-12(K88ab) to adhere to immobilized porcine small intestine mucus was examined. E. coli K-12(K88ab) but not the isogenic E. coli K-12 strain was found to adhere readily to immobilized crude mucus but not to bovine serum albumin. The adhesion of E. coli K-12(K88ab) was inhibited in a specific fashion by anti-K88 antiserum. Adhesion was also inhibited by pretreatment of receptor-containing crude mucus preparations with sodium metaperiodate or proteolytic enzymes. Removal of glycolipids from crude mucus by chloroform-methanol extraction did not affect the ability of E. coli K-12(K88ab) to bind to mucus preparations. Adsorption of crude mucus preparations with K88ab fimbriae but not type 1 fimbriae resulted in the removal of K88-specific receptors. Analysis of the pelleted fimbriae-receptor complex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, together with gel filtration chromatography of crude mucus preparations, suggest that the K88-specific receptor present in porcine small intestine mucus is a 40- to 42-kDa glycoprotein.