Low doses of 6-mercaptopurine (6MP) were used for the treatment of inflammatory bowel disease, and 20-30 mg/day was found to be effective for patients with ulcerative colitis who were corticosteroid-dependent or corticosteroid-resistant. Corticosteroid was tapered in 20 of 21 patients with ulcerative colitis. Of 15 patients who were refractory to conventional therapy, 11 responded to 6MP treatment. The same doses of 6MP were given to patients with Crohn's disease who were corticosteroid-dependent or who had associated fistula. Treatment with 6MP did not influence the changes in colonic or ileac lesions in Crohn's disease. However, the fistulas were closed or improved in 70% of 10 patients by 6MP treatment. The adverse effects of small doses of 6MP were minimal. These results confirm that immunosuppressive agents are effective for patients with inflammatory bowel disease. In a rat colitis model induced by immunization with trinitrobenzene (TNB), we used anti-CD4 monoclonal antibodies to prevent colonic inflammation; these antibodies were effective for this colitis model, suggesting that a novel therapy targeting CD4 intestinal lymphocytes may be feasible in the treatment of Crohn's disease.