Most arthritic conditions are characterized by chronic inflammation, resulting in secondary changes in serum biochemistry. In an attempt to profile different mechanisms of inflammation which might account for the clinical diversity of rheumatic diseases, we have measured C-reactive protein (CRP), plasma viscosity, serum histidine and total serum sulphydryl in 259 patients with rheumatoid arthritis (RA), 84 with ankylosing spondylitis (AS), 76 with osteoarthritis, 69 with psoriatic arthritis, 34 with systemic lupus erythematosus (SLE), 36 with Reiter's syndrome and 121 normal controls. The most extreme abnormalities were seen in rheumatoid arthritis and the least in osteoarthritis. The seronegative spondarthritides and SLE occupied a midway position, emphasizing a correlation between biochemical abnormality and severity of inflammation. A low serum histidine characterized both RA and SLE. The former was more likely to be associated with a raised CRP. Plasma viscosity was characteristically raised in psoriatic arthritis and CRP in AS.