DISEASE CHARACTERISTICS: Histologically proven metastatic, locally recurrent, or surgically unresectable colorectal carcinoma Measurable disease No CNS metastasis

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: WHO 0-2 Life expectancy: Not specified Hematopoietic: Absolute neutrophil count at least 2,000/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.5 mg/dL SGOT no greater than 2.5 times normal (5 times normal if known liver metastases) Renal: Creatinine no greater than 1.5 mg/dL Other: No severe concurrent infection Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No more than 1 prior chemotherapy regimen Prior fluorouracil-based regimen for advanced colorectal cancer allowed Prior fluorouracil with radiotherapy not considered therapy for advanced disease At least 4 weeks since prior chemotherapy (8 weeks for mitomycin or nitrosoureas) and recovered Endocrine therapy: No concurrent steroids except for antiemesis or as replacement therapy Radiotherapy: See Chemotherapy No prior radiotherapy to more than 30% of bone marrow Surgery: See Disease Characteristics At least 4 weeks since prior surgery unless fully recovered Other: No concurrent vitamin supplements containing folic acid No other concurrent anticancer therapy