DISEASE CHARACTERISTICS: Diagnosis of metastatic colorectal adenocarcinoma incurable by surgery or not amenable to radiotherapy with curative intent Progressive disease no more than 3 months after completion of a prior weekly irinotecan, fluorouracil, and leucovorin calcium chemotherapy regimen At least 1 measurable lesion At least 20 mm in at least one dimension No known CNS metastases or carcinomatous meningitis

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Absolute neutrophil count greater than 2,000/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 0.5 mg/dL above upper limit of normal (ULN) AST no greater than 5 times ULN Renal: Creatinine no greater than 1.5 times ULN OR Creatinine clearance greater than 60 mL/min Cardiovascular: No unstable angina No symptomatic congestive heart failure No serious uncontrolled cardiac arrhythmia Other: No active or uncontrolled infection No evidence of other serious illness No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or noninvasive carcinomas Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent sargramostim (GM-CSF) Chemotherapy: See Disease Characteristics At least 4 weeks since prior chemotherapy No more than 1 prior chemotherapy regimen for advanced colorectal cancer Prior adjuvant chemotherapy allowed Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics No prior radiotherapy to more than 25% of bone marrow Surgery: See Disease Characteristics At least 4 weeks since prior major surgery