Cartilage proteoglycans were measured, by the use of an enzyme-linked immunosorbent assay, in synovial fluids obtained from 109 unselected patients attending an outpatient rheumatology clinic because of inflammation of the knee. The content of proteoglycans in synovial fluid was inversely related to the degree of joint destruction shown on X-ray. The proteoglycan concentrations in knee-joint exudates were higher in patients with reactive arthritis than in patients with rheumatoid arthritis having synovitis of corresponding duration and lacking radiological evidence of cartilage destruction. The measurement of proteoglycan levels may therefore be useful in differentiating between the two conditions.