Expression of decay-accelerating factor (DAF, CD55), a complement-regulatory glycoprotein, is enhanced in colorectal-cancer (CC) cells and colonic epithelium in ulcerative colitis (UC), and stools from these patients contain increased amounts of DAF. Carbohydrate chains of glycoproteins are often altered during malignant transformation or inflammation. In this study, we investigated whether DAF molecules in patients with CC and those with UC differ with respect to oligosaccharide side chains. We analyzed DAF in stools and homogenates of colonic-tissue specimens obtained from patients with CC or UC using solid-phase enzyme-linked assay and Western blotting for reactivity with the lectins Ulex europaeus agglutinin I (UEA-I), wheat-germ agglutinin, peanut agglutinin, and concanavalin A. UEA-I bound to DAF in stools from patients with UC but not in that from the stools of CC patients, as demonstrated on the solid-phase enzyme-linked assay (P <.05, Mann-Whitney U test) and Western blotting. Binding of UEA-I was specifically inhibited by the addition of fucose. The difference in UEA-I reactivity with DAF was observed also in colonic-tissue homogenates from patients with UC and those with CC. DAF expressed in the mucosa and excreted into the stools of UC patients is different from that expressed in CC with regard to UEA-I reactivity. Future studies should be directed toward determining whether a qualitatively unique isoform of DAF is present, of which sugar chains are specific to CC in UC patients.