A recombinant (r)65-kD protein from Mycobacterium leprae, at levels far in excess of those present in whole mycobacteria, was unable to induce arthritis. Even when combined with a synthetic adjuvant, CP20961, to mimic the peptidoglycan adjuvant component of the mycobacterial cell wall, the r65-kD protein failed to induce arthritis. Pretreatment with as little as 1 microgram r65-kD protein protected rats against arthritis induced by M. tuberculosis, but this r65-kD protein was markedly less able to protect against arthritis induced by the synthetic adjuvant, CP20961, or type II collagen. The r65-kD protein appears, therefore, to produce an antigen-specific protection against arthritis induced by bacterial cell walls containing the 65-kD protein. Such protection can be overcome, however, by arthritogenic T lymphocytes, suggesting that protection occurs by preventing clonal proliferation of autoreactive T lymphocytes that are induced by the adjuvant properties of mycobacterial cell walls. How the r65-kD protein abrogates this particular adjuvant activity, and the nature of the arthritogenic self antigen(s), remain to be elucidated.