Synovial fluids from 106 patients with various types of arthritis were examined for the presence of conversion products of C3 by means of crossed antigen-antibody electrophoresis and for products of factor B by immunoelectrophoresis. C3 conversion was found in all 15 fluids from patients with seropositive rheumatoid arthritis, in 11 of 15 with seronegative rheumatoid arthritis, in the majority with probable and possible rheumatoid arthritis, juvenile rheumatoid arthritis, SLE, pseudogout, gout, Reiter's syndrome, and frequently in other arthritides studied, but in only one of 15 with degenerative arthritis. In 53 synovial fluids a single C3 conversion peak was seen in addition to the native protein and in 18 others two conversion peaks were present. In many synovial fluids showing conversion whole-complement titers and C3 protein concentrations were normal. In both rheumatoid arthritis and crystal synovitis the per cent of C3 conversion, as estimated by planimetry, correlated with synovial fluid leukocyte counts, Factor B conversion was found in 31 fluids and usually occurred in fluids also showing C3 conversion. The findings indicate that in vivo activation of components of the classical and alternative considered mediated by immune complexes. Activation is also commonly present in a wide variety of other inflammatory arthritides and ofter would not be recognized by measuring only concentrations of hemolytic whole complement or C3 by immunodiffusion. The positive association between C3 conversion and synovial fluid polymorphonuclear leukocytes suggests that chemotactic factors generated from complement may be responsible for the attraction of leukocytes into the synovial space in these diseases.