A case of ulcerative colitis (UC) associated with anti-erythrocyte antibodies is reported. A 27-year-old male visited our department because of a relapse of UC, and his condition was diagnosed as severe and extensive colitis. He was vigorously treated with prednisolone (50 mg/day i.v.) and TPN, but no response was obtained. Subtotal colectomy with ileostomy was performed on the 22nd hospital day, and a proctosigmoidectomy was performed on him six months later. Preoperative blood type and screen disclosed that his blood was not cross-matched either on the forward or reverse typing. His ABO blood group could not be identified as "B", as determined upon admission, and his Coombs' test was positive. Further studies revealed that his Rh group was CcDEe. Anti-I antibody was detected in his serum and eluate from erythrocytes were identified as anti-E and c. Titers of the antibodies were strongest in the preoperative period and became weak following colectomy, but the antibodies did not disappear even after the large bowel was totally removed.