The determination of the relative abilities of 11 chelating agents to enhance the urinary and fecal excretion of uranium when administered 10 min after uranyl acetate dihydrate (UAD) in mice showed that the most effective of these were Tiron, desferrioxamine, and 1,2-dimethyl-3-hydroxypyrid-4-one. An increase in the interval between UAD administration and that of the chelating agent drastically reduces the net mobilization of the uranium by the chelating agents examined. When given shortly after UAD, Tiron produced the greatest reduction in renal and bone levels of uranium. None of the chelating agents were able to affect the bone levels of uranium when administered 24 hr or more after the administration of the UAD.