Measurements were made of the IgG in synovial fluid which is synthesized by the intact rheumatoid synovium. In five rheumatoid subjects 12–24% of the IgG present in their synovial fluid was derived from local production. No evidence for local production was found in patients with degenerative arthritis or Reiter's syndrome. It was estimated that as much as 95 mg of IgG was produced by the synovium of a single knee joint daily. Rheumatoid inflammation is associated with the presence of large numbers of white blood cells, predominantly polymorphonuclear, in the articular cavity. Each day as many as one billion white cells enter the articular cavity to participate in this inflammatory response. Factors causing the directed migration of polymorphonuclear leukocytes were found in the majority of rheumatoid effusions studied. The chemotactic activity is, in large part, related to the fifth (C5) and sixth (C6) components of human complement. Physical-chemical techniques indicate that the activity is attributable to C 567 and C5a, a cleavage product of C5. In addition to the presence of preformed chemotactic factors, more than half of the rheumatoid fluids contain an enzyme capable of generating chemotactic activity from the fifth component (C5) of human complement. This information supports the concept that rheumatoid joint inflammation is an example of immune complex disease in which a significant proportion of the immunoreactants are derived from local production.