PATIENT CHARACTERISTICS: Age: Over 18 Performance status: SWOG 0-2 Hematopoietic: WBC at least 4,000/mm3 Platelet count normal Hepatic: Bilirubin no greater than 2 times upper limit of normal (ULN) SGOT no greater than 2 times ULN Alkaline phosphatase no greater than 2 times ULN Renal: Not specified Other: No chronic ulcerative colitis No other serious medical illness that would preclude protocol therapy No psychiatric condition that would preclude informed consent No noncolorectal malignancy within 5 years except: Adequately treated nonmelanomatous skin cancer Adequately treated carcinoma in situ of the cervix Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception