DISEASE CHARACTERISTICS: Histologically confirmed carcinoma of the colon or rectum with radiologically confirmed metastases or clinically confirmed incurability No hepatic or CNS metastases Progression following fluorouracil (5-FU) alone or with a modulator (e.g., leucovorin, PALA, levamisole, interferon) Measurable disease outside prior radiotherapy fields The following are not considered measurable: Pleural effusion or ascites Osteoblastic lesions or evidence of disease on bone scan alone Progressive irradiated lesions alone Bone marrow involvement Brain metastases Malignant hepatomegaly by physical exam alone Chemical markers (e.g., CEA)