Complement activation in seropositive and seronegative rheumatoid arthritis. 125I-C1q binding capacity and complement breakdown products in serum and synovial fluid.

1. The detection and quantitation of immune complex-like material in synovial fluid and in serum from patients with joint diseases was done through the measurement of the capacity to bind radiolabeled C1q. It was found that 65% of synovial fluid samples from seropositive or seronegative RA patients had a high C1q binding capacity as compared to other joint diseases. Immune complex-like material was also detected in 63% of serum samples from seropositive RA patients. 2. The existence of C3 or C3PA breakdown products in synovial fluid from most of the synovial fluids from RA patients probably reflects an activation of the complement system occurring in both forms of the disease. C3PA breakdown products were never found in degenerative or post-traumatic joint diseases and only occasionally in other inflammatory arthritis. Apart from their pathogenic significance, these results may have some interest for the clinical investigation of patients with joint diseases.