Caeruloplasmin (Cp), prealbumin and alpha 2-macroglobulin (alpha 2-M) concentrations in serum were compared in rheumatoid arthritis (RA), osteoarthrosis, ankylosing spondylitis, psoriatic arthritis, Reiter's syndrome, Behçet's syndrome, SLE and normal controls. Cp was significantly elevated (p less than 0.01) in all disease groups except for Reiter's syndrome and SLE. Prealbumin was only significantly depressed (p less than 0.01) in RA. The most notable elevation in alpha 2-macroglobulin occurred in Reiter's syndrome. However, these differences were generally still within the normal range, and hence these proteins are unlikely to offer alternative indices of disease activity suitable for monitoring the the progress of seronegative arthropathies.