Prealbumin was shown to be a sensitive indicator of disease activity in a prospective study of 21 patients with active ankylosing spondylitis (AS) who were treated with 3 intravenous pulses of methylprednisolone and its concentration was found to change at a different rate to C-reactive protein (CRP). In those diseases in which CRP concentration rises with active disease, i.e., rheumatoid arthritis, AS and Crohn's disease, prealbumin fell, but in those diseases in which CRP rises only slightly, i.e., systemic lupus erythematosus, progressive systemic sclerosis and ulcerative colitis, there was nevertheless a fall in serum prealbumin, indicating that there was an acute phase response occurring. Fever, arthritis and infection were the only disease manifestations that were associated with an elevated CRP in both groups of diseases. There is therefore more than one signal for an acute phase response depending on the nature of the disease pathology.