The effects of treatment with second-line antirheumatic drugs and cytotoxic agents on the severity of experimental monoarticular arthritis in BALB/c mice have been investigated. The arthritis was assessed histologically in terms of synovitis and erosions of cartilage and bone. Azathioprine (20 mg/kg) and sulphasalazine (10-30 mg/kg oral; 30-100 mg/kg i.p.) produced significant suppression of synovitis and erosions when administered daily for 4 weeks commencing 2 weeks after induction of the arthritis. Dapsone (1-10 mg/kg) and to a lesser extent methotrexate (2 mg/kg) produced some suppression of erosive disease when administered daily for 4 weeks commencing 2 weeks after induction of the arthritis but this failed to reach statistical significance. Chloroquine, D-penicillamine and sodium aurothiomalate all failed to have any effect on the disease with any of the treatment schedules used. Auranofin had no effect on the disease when treatment commenced 2 weeks after intra-articular injection but produced variable suppression at high doses when administered from the day of intra-articular injection.