Iron dextran (molecular weight 7,000) diffuses rapidly from the joint cavity through the synovium, along lymphatics and extracellular tissue spaces; articular cartilage is impermeable to iron dextran. There is also rapid cellular uptake by synovial lining cells, particularly of the vacuolar type; endoplasmic reticulum-containing lining cells rarely take up iron dextran. Cellular uptake is probably effected by pseudopodial folds projecting from the cell surface and enclosing extracellular material. Cells containing iron may degenerate and be ingested by phagocytes, and this may account for the concentration of iron in a smaller proportion of cells on or below the synovial surface in the later stages. At 6 to 18 hours after injection there is a mild inflammatory reaction and some synovial proliferation; from this stage onwards intracellular iron occurs in the form of haemosiderin. Granules of haemosiderin are present in the synovium 3 months after injection and possibly longer.