Adjuvant arthritis, induced by Mycobacterium butyricum, is an experimental immunopathy that shares many features of human rheumatoid arthritis and, as such, is one of the most widely used models for studying the anti-inflammatory activity of compounds. In rats with adjuvant induced arthritis, IgG antibodies to M. butyricum have been detected and autoantigens that cross react with mycobacteria may be involved in the pathogenesis of adjuvant arthritis. In this study, the anti-inflammatory and immunosuppressive activities of two cyclooxygenase-2 selective inhibitors, flosulide and L-745,337, at doses of 0.1, 1 and 5 mg/kg/day, were examined in adjuvant arthritic rats. After 14 days of treatment, a clear dose-dependent inhibition of plantar edema was seen for both flosulide (ID50 lower than 0.1 mg/kg) and L-745,337 (ID50 = 0.4 mg/kg). Plasma levels of IgG anti-M. butyricum antibodies were also decreased by both drugs. In each case the maximal immunosuppressive effect was observed at doses lower than 5 mg/kg. The non-selective COX-2 inhibitor, indomethacin (1 mg/kg) decreased paw edema by 65% and the levels of IgG anti-M. butyricum by 45%. Neither cyclooxygenase selective inhibitors nor indomethacin decreased the delayed hypersensitivity reaction induced by M. butyricum. Thus, in vivo inhibition of COX-2 inhibited articular swelling and also the humoral immune response to Mycobacterium.