The uptake of metallic mercury by ferritin was studied. The results were; 1) Ferritin purchased from Sigma Company (No.F4503, cadmium content less than 1%) after dialysis showed 3 times the mercury uptake compared with ferric ion with H(2)O(2) and 10(3) times compared with Fe(OH)(3) with H(2)O(2). Mercury uptake by ferritin with H(2)O(2) and ascorbic acid decreased 3.4 times compared with that without ascorbic acid (with H(2)O(2)). 2) Mercury uptake by further purified ferritin showed 0.1446 μg/umole protein with H(2)O(2), 0.1133 μg/umole protein with H(2)O(2) and ascorbic acid, and 0.1345 μg/umole protein without H(2)O(2) and ascorbic acid. 3) Iron in ferritin has the composition (FeO.OH)8(FeO.OPO(3)H(2)). The mercury uptake of hydrous ferric oxide using Bultal (Chondroitin sulfate･ iron colloid) as an iron compound showed a low value per Fe at each concentration. 4) Catalase activity of ferritin was negligible by the perborate method.