PRIOR CONCURRENT THERAPY: Biologic therapy: Prior murine monoclonal antibody allowed Chemotherapy: No prior irinotecan At least 4 weeks since prior chemotherapy and recovered (8 weeks since nitrosourea, mitomycin or 90Y-hMN-14) Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy to index lesion and recovered No prior radiotherapy to greater than 25% of red marrow (pelvic field radiation as adjuvant therapy for rectal cancer allowed) No prior radiotherapy to maximum tolerated dose to any critical organ (e.g., lung, liver, or kidney) Surgery: At least 4 weeks since major surgery