Clq levels in sera of adult patients with rheumatoid arthritis (RA) were found to be significantly higher than corresponding levels of normal subjects (p less than 10(-3)) The increase in Clq observed in RA was not seen in systemic lupus erythematosus, chronic polyarticular gouty arthritis, ankylosing spondylitis, Paget's disease of bone, or Hodgkin's disease. Clq levels were determined both chemically (calculated) from the protein-bound hydroxyproline content of the euglobulin fraction) and by radial immunodiffusion. Although the estimates of Clq by these 2 methods did not agree, the increase of Clq in RA was found by the use of either method.