Adjuvant arthritis is an experimental disease of rats induced by immunization to antigens of Mycobacterium tuberculosis. Our observation that arthritis could be induced in irradiated rats by the A2 line of T lymphocytes in the absence of mycobacterial antigens suggested that adjuvant arthritis is an autoimmune disease. Moreover, the A2 line could be used to vaccinate unirradiated rats against the subsequent induction of adjuvant arthritis by active immunization to Mycobacteria. In the present study we found that thymus cells obtained from A2 vaccinated rats could transfer resistance to adjuvant arthritis to naive rats. This indicates that the mechanism of resistance induced by A2 vaccination is probably immunological and involves thymus-derived lymphocytes.