DISEASE CHARACTERISTICS: Histologically confirmed advanced or metastatic adenocarcinoma of the colon and rectum Surgically unresectable disease Measurable disease Disease outside the prior radiotherapy port and/or progressive disease within the previously irradiated volume

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: Not specified Hematopoietic: WBC at least 3,000/mm3 Platelet count at least 100,000/mm3 No hemorrhagic events within the past 6 months Hepatic: Bilirubin normal SGOT normal INR no greater than 1.5 Renal: Creatinine no greater than 1.5 times upper limit of normal Cardiovascular: No thromboembolic events within the past 6 months Pulmonary: No evidence of pneumonia Other: No known hypersensitivity to recombinant humanized murine monoclonal antibodies No evidence of significant active infection (e.g., peritonitis or wound abscess) Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent sargramostim (GM-CSF) Chemotherapy: At least 12 months since prior fluorouracil-based adjuvant chemotherapy No prior adjuvant irinotecan Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics At least 2 weeks since prior radiotherapy and recovered Surgery: See Disease Characteristics Other: No prior therapy for advanced disease No concurrent therapeutic anticoagulation except for low-dose coumadin for maintenance of indwelling catheters