OUTLINE: This is a randomized, multicenter study. Patients are stratified according to type of prior surgery (abdominoperineal resection vs anterior resection), nodal status (N0 vs N1 vs N2-3), depth of tumor invasion (T1-2 vs T3 vs T4a vs T4b), time from surgery to study entry (20-45 days vs 46-70 days), participating center, and performance status (0-1 vs 2). Patients are randomized to one of three treatment arms. Arm I: Patients receive fluorouracil (5-FU) IV on days 1-5 and 29-33. 5-FU is then given as a continuous infusion beginning on day 57 and continuing concurrently with radiotherapy for 5 weeks. Following a 28 day break from treatment patients receive 5-FU IV on days 1-5 of a 28 day course. Postradiotherapy treatment repeats for a total of 2 courses in the absence of disease progression or unacceptable toxicity. Arm II: Patients receive 5-FU IV continuously on days 1-42. 5-FU and radiotherapy are then administered as in arm II. Arm III: Patients receive leucovorin calcium (CF) IV followed by 5-FU IV on days 1-5 and 29-33. Patients also receive oral levamisole twice daily on days 1-3, 15-17, 29-31, and 43-45. CF IV and 5-FU IV are then given on days 57-60 and 85-88 concurrently with radiotherapy. Following a 28 day break from treatment patients receive CF IV and 5-FU IV on days 1-5 and 29-33 and oral levamisole twice daily on days 1-3, 15-17, 29-31, and 43-45 in the absence of disease progression or unacceptable toxicity. All patients receive radiotherapy 5 days per week for 5 weeks starting on day 57. Patients are followed every 4 months for 2 years, then every 6 months for 4 years, and then annually until death.