Total body irradiation followed by bone marrow transplantation was found to be an effective treatment for adjuvant arthritis induced in rats. This treatment is most effective when applied shortly after the clinical manifestation of arthritis--i.e., 4-7 weeks after administration of Mycobacterium tuberculosis. Transplantation of bone marrow at a later stage results in a limited recovery, in that the inflammatory reaction regresses but the newly formed excessive bone is not eliminated. Local irradiation of the affected joints had no effect on the disease. It could also be excluded that the recovery of arthritis following marrow transplantation is due to lack of available antigen. Transplantation of syngeneic bone marrow is as effective as that of allogeneic bone marrow from a rat strain that is not susceptible to induction of adjuvant arthritis. The beneficial effect of this treatment cannot be ascribed to the immunosuppressive effect of total body irradiation, since treatment with the highly immunosuppressive drug Cyclosporin A resulted in a regression of the joint swelling but relapse occurred shortly after discontinuation of the treatment.