Complement activation was evaluated in plasma and synovial fluid from patients with rheumatoid arthritis. The activation fragment C3dg and the fluid-phase terminal complement complex were used as indicators of initial and terminal activation, respectively. Considerable activation of the whole complement cascade was demonstrated in most synovial fluid samples and in one-third of the plasma samples. No correlation was found between the level of activation products in synovial fluid and the level in plasma. Therefore, both compartments must be examined in order to evaluate local and systemic complement activation in rheumatoid arthritis.