PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: Prior chemotherapy with a fluoropyrimidine alone or in combination with an accepted modulating agent (leucovorin, levamisole, methotrexate, interferon, PALA, or hydroxyurea) required Prior chemotherapy with uracil mustard and tegafur (UFT) allowed No concurrent chemotherapy allowed during study Endocrine therapy: Not specified Radiotherapy: No whole pelvic or abdominal radiation therapy allowed Surgery: No colectomy, coloanal anastamosis, abdominoperineal resection, or colostomy allowed Other: No antidiarrheal agents (e.g., diphenoxylate hydrochloride, elixir paregoric, opium tincture or tincture of belladonna, or kaolin) during study No cyclosporine allowed