To assess the possible participation of cellular immune mechanisms in the pathogenesis of rheumatoid arthritis (RA) in vitro studies of the blastogenicity of rheumatoid and non-rheumatoid synovial fluids for human peripheral blood lymphocytes were conducted. In autologous cultures it was found that 13 of 19 rheumatoid fluids induced significant lymphocyte blastogenesis, whereas only 1 of 13 nonrheumatoid fluids induced such a response. In allogeneic cultures rheumatoid fluids induce significant blastogenesis of RA lymphocytes in 18 of 23 experiments, and of non-RA lymphocytes in 8 of 18 experiments. By contrast, nonrheumatoid fluids induced significant blastogenesis of RA lymphocytes in 2 of 13 experiments, and of non-RA lymphocytes in 1 of 14 experiments. The blastogenicity of fluids was found to correlate significantly with the presence therein of immunofluorescent intracellular inclusions of immunoglobulin and complement. These studies support the concept that the presence of immune complexes in the majority of rheumatoid synovial fluids might render the latter blastogenic for human lymphocytes in vivo, thereby perpetuating rheumatoid synovitis.