Using an immunoradiometric assay with two monoclonal antibodies plasma C9 levels were measured in 49 patients with rheumatoid arthritis, eight patients with Behçet's disease, six patients with ankylosing spondylitis, nine patients with psoriatic arthropathy, six patients with systemic lupus erythematosus, five patients with erosive osteoarthritis and four patients with acute gout. Plasma C9 levels were also followed sequentially in four patients with Behçet's disease to show fluctuation during disease activity. The upper limit of normal for plasma C9 was 90 mg/l, 2 standard deviations above the known mean value. Significantly raised levels were found in active rheumatoid arthritis (119.7 +/- 28.3), Behçet's disease (177.9 +/- 26.2) and ankylosing spondylitis (116.1 +/- 23.1). In Behçet's disease the raised C9 levels normalized during disease quiescence suggesting that C9 is a good monitor of disease activity. CRP and ESR values were also measured. In rheumatoid arthritis C9 was more consistently elevated in active disease than CRP or ESR. As there was no association between these measures on mathematical analysis it was concluded that plasma C9 is a better indicator of disease activity in rheumatoid arthritis.