Resistance (low dose tolerance) to adjuvant arthritis was induced by intradermal immunization with 10 micrograms Mycobacterium tuberculosis administered 5 and 3 weeks before induction of arthritis. With the purpose of determining phenotypes of cells which participate in the maintenance of the induced resistance to adjuvant arthritis, tolerized rats were treated with two different anti-T-cell monoclonal antibodies. In tolerized rats, it was shown that anti-CD8 (OX8) antibodies, which caused an elimination of CD8+ lymphoid cells as determined by immunofluorescence analysis, made the rats responsive to an arthritogenic challenge with mycobacteria. Nine of 19 (47.4%) rats developed the disease as compared with 2 of 18 (11.1%) (P less than 0.05) in the control antibody-treated group. Also, in vivo treatment with anti-CD5 (OX19) monoclonal antibodies made the rats responsive to an arthritogenic challenge with mycobacteria. Nine of 15 (60%) anti-CD5-treated rats developed the disease as compared with 2 of 18 (11.1%) (P less than 0.01) rats in the control group. Immunofluorescence analysis performed after anti-CD5 treatment showed a reduction of staining of CD5+ cells as well as a down-regulation of the staining intensity of CD5 cell surface receptors on the remaining CD5+ cells. These data indicate that CD8+- as well as CD5+ cells participate in the maintenance of low dose tolerance to adjuvant arthritis.