A highly specific antiserum against rat liver metallothionein (MT) was raised in a Japanese white rabbit. Using this anti-MT antiserum, we found that MT was localized in the nuclei as well as in the cytoplasm of hepatocytes in newborn rats. Since it is known that these cells are growing actively, we suspected that there was a relationship between the localization of MT in cell nuclei and the cell proliferation. Therefore, the induction and subcellular localization of MT were examined in rat liver remaining after 70% removal. MT was induced in the remnant liver rapidly after the hepatectomy, its concentration being about 80-fold higher than that of the intact liver. Flow cytometric analysis revealed that MT was translocated into the nuclei from the cytoplasm of hepatocytes during liver regeneration after partial hepatectomy. The highest MT level in the nuclei was observed 24 h after hepatectomy. MT-stained positive nuclei were in S to G2M phases of the cell cycle of regenerating hepatocytes, and the nuclei in G1 phase were not stained with anti-MT antiserum. The increase in hepatic MT levels did not directly cause MT translocation into the nuclei. These results suggested that MT was a cell cycle-dependent, nuclear protein.