The effectiveness of some chelating agents to mobilize cadmium from Chinese hamster ovary cells after chronic exposure (20 hr), as well as from cytosolic metallothionein, was studied. In the first protocol the most effective substance was 2,3-dimercaptopropanol, followed by 2,3-dimercaptopropane-l-sulfonate and 2,3dimercaptosuccinic acid, whereas CaNa3-diethylenetriamine pentaacetic acid >( 51-120 showed less effect. Simultaneous incubation of cells with cadmium and the chelating agent resulted in a different order of effectiveness: CaNa3 DTPA prevented cadmium uptake almost totally, 2,3-mercaptopropanol by 75% and 2,3dimercaptopropane-l-sulfonate by 35%. Neither CaNmdiethylenetriamine pentaacetic acid )< 51-120 nor 2,3-dimercaptosuccinic acid had altered the distribution of cadmium between the cytosolic protein fractions after a 2 hr incubation of cells, whereas after this period, 2,3-dimercaptopropanol had removed all cadmium from metallothionein, and 2,3dimercaptopropane-l-sulfonate about 50%. None of the chelating agents had reduced the amount of Cd bound to high molecular weight proteins. In the cell-free system, 2,3-dimercaptopropanol and 2,3-dimercaptopropane-l-sulfonate were equally effective and removed all cadmium from metallothionein within ten minutes. CaNm-diethylenetriamine pentaacetic acid X 51-120, however, even after 60 min, had removed only 50% of the cadmium. The remaining cadmium was found distributed to the high molecular weight and lower molecular weight protein fractions.