The origin of erythrocyte metallothionein (MT) in cadmium (Cd) administered mice and its fate were studied in connection with erythrocyte kinetics. It was shown that Cd-MT in the erythrocyte is synthesized in the precursor cells of the erythropoietic tissues, and Cd-MT containing erythrocytes come into the circulation after maturation. In regard to the degradation, it was found that Cd-MT is stable in the erythrocyte, but decomposed in the spleen or liver with the breakdown of erythrocyte when its life span is over.