Metallothionein from livers of mice was isolated by gel chromatography and isoelectric focusing. One of two forms thus obtained contained 32 percent cysteine. This form, labeled in vitro with 109Cd, was injected intravenously in mice, and the distribution of 109Cd was studied. Animals killed after 4 hrs had over 80 percent of the injected dose in the kidneys. Protein obtained after gel chromatography, containing both forms of cadmium-binding protein, was also labeled in vitro with 109Cd and injected intravenously. Animals killed 4 hrs after injection had 50 percent of the injected dose in the kidneys. Whole-body measurements and wholebody autoradiography demonstrated that approximately 40-60 percent of the injected dose had been excreted in urine. The results show a selective accumulation of metallothionein-bound cadmium in the kidney and indicate possible differences in distribution and excretion of cadmium depending on binding to different forms of low molecular weight cadmium-binding proteins.