Synthetic triclinic calcium pyrophosphate dihydrate crystals, uniformly labeled with *'Sr and 45Ca, were injected into the knee joints of 2 normal adult rabbits and 2 rabbits previously injected repeatedly with autologous blood. The "half clearance time" of the injected crystal mass was 20.4 and 19 days from control joints, nearly identical to previously reported values in 6 rabbits (19.1 f 1.4), and 28.8 and 34 days from the joints injected with blood, a significant difference (P < 0.05).Iron stains showed hemosiderin granules in the superficial synovium in these joints. Electron microscopy showed crystals with a molar calcium/phosphorus ratio of 1.0 and particles containing iron within synovial cells. We hypothesize that the decreased clearance rate from hemosiderotic synovium is due to inhibition of one or more intracellular pyrophosphatases by iron.A method for determination of the synovial clearance rates of radiolabeled synthetic triclinic cal-