Specimens of joint fluids and sera from 24 patients with rheumatoid arthritis (RA) and 31 patients with other rheumatic conditions were tested by a passive hemagglutination assay for the presence of antibodies to human native collagens and to their respective chains. A large percentage of the RA patients displayed a high occurrence of anticollagen antibodies in the serum, and the titers and occurrence of these antibodies were even higher in the respective synovial fluids. Synovial fluids, but not sera, of three osteoarthritic patients possessed antibodies to collagen chains. Patients with gouty arthritis, pseudogout, and/or traumatic effusions had no anticollagen antibodies. © 1976.