Synovitis was produced in guinea-pigs previously immunized with bovine γ-globulin and Freund's complete adjuvant, by injection of bovine γ-globulin and tuberculin PPD into the knee joints. In animals given prior intravenous injections of tritiated thymidine and colloidal carbon, up to 40 per cent of the local accumulations of mononuclear cells 48 hours after antigen challenge were tritium-labelled as seen by autoradiography. These inflammatory cells are considered to be of haematogenous origin. A smaller percentage of cells was carbon-labelled. The percentage of tritium-labelled cells decreased with duration of the granuloma following antigen challenge. In other animals, synovial inflammatory cells were labelled by intra-articular injection of tritiated thymidine at different times following the establishment of synovial inflammation by injection of antigen. The highest proportion of cells labelled was 17 per cent, but when label was administered as late as 60 days after the inflammatory stimulus, only about 3 per cent of cells were labelled. Grain counts of cells labelled by local tritium administration or by short-term incubation of slices in vitro showed approximately 30–100 grains per nucleus. Sequential study of locally labelled synovia showed that such high-grain cells were still present 12 days after tritium administration, suggesting that at least some of the mononuclear cells were relatively long-lived.