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