Metallothioneins (MTs) are heavy metal-binding proteins which play key roles in protection against heavy metal toxicity, and therefore it is toxicologically important to understand the regulatory mechanism of MT production. In the present work, I isolated mouse cell clones which express the bacterial neo gene (G418-resistance gene) under the control of an MT gene regulatory sequence; such cells will serve as parents for the isolation of mutants defective in MT gene regulation. C-127 and L-929 cells were transformed with a plasmid construct containing the regulatory region of the human MTIIA gene linked to the neo structural gene, and stable G418-resistant transformants were selected. When Zn was given during the selection, the number of the resulting colonies increased significantly, suggesting a Zn-induced expression of the introduced neo gene. The C-127 transformants selected in the presence of Zn have several copies of the neo sequence in the chromosome, and express a neo message in a Zn-inducible manner. When Zn was removed from the medium, G418 damaged the cells. These cell clones are expected to be useful for isolating mutants that will help further understanding of the control mechanism of MT gene expression.