Two proteins in the properdin system, properdin and factor B, and complement components C4 and C3 were measured by radial immunodiffusion in serums and synovial fluids from 21 patients with rheumatoid arthritis (RA) and positive tests for rheumatoid factor, 9 patients with seronegative RA, and 10 with degenerative joint disease. In addition to depressions of synovial fluid C4 which correlated with lowered C3 in seropositive RA, consistent with activation of the classic pathway, low synovial fluid levels of factor B and properdin in seropositive RA indicate intraarticular activation of the properdin pathway as well.