OUTLINE: This is a randomized, multicenter study. Patients are stratified according to center, T stage (T1 vs T2 vs T3 vs T4a), N stage (N0 vs N1 vs N2-3), and time from surgery until registration (21-28 vs 29-35 days). Patients are randomized to one of two treatment arms. Arm I: Patients receive leucovorin calcium IV immediately followed by fluorouracil IV on days 1 through 5 of weeks 1, 5, 9, 14, 19, and 24, and oral levamisole 3 times a day for 3 days on days 1-3. Courses repeat every 2 weeks for 6 months. Arm II: Patients receive levamisole as in arm I plus fluorouracil by continuous IV infusion for 8 weeks. Treatment with fluorouracil repeats every 10 weeks for 3 courses. Treatment continues in the absence of disease progression or unacceptable toxicity. Patients are followed at 9 months and 12 months, then every 6 months for 4 years, and then annually thereafter.