Metallothionein (MT), a low-molecular-weight, cysteine-rich, metal-binding protein, has been implicated in the detoxification of Cd. However, whether MT protects against the cellular toxicity of other metals has not been examined thoroughly. This study was therefore designed to determine the effects of Zn-induced MT on the toxicity of seven metals in rat primary hepatocyte cultures. Hepatocytes were grown in monolayer culture for 22 hr and subsequently treated with ZnCl2 (100 microM) for 24 hr which produced a 15-fold increase in MT concentration. Following Zn pretreatment, hepatocytes were exposed to various concentrations of Ag, Co, Cu, Hg, Ni, Pb, or Zn for 24 hr. Cytotoxicity was assessed by enzyme leakage and loss of intracellular K+. The toxicity of all seven metals was significantly less in the Zn-pretreated cells. Zn pretreatment had no appreciable effect on the hepatocellular uptake (1-24 hr) of 110Ag or 203Hg, but markedly altered their subcellular distribution, with metals accumulating more in the cytosol and less in the nuclear, mitochondrial, and microsomal fractions. In the cytosol of control cells, the metals were bound mainly to high-molecular-weight proteins whereas in the Zn-pretreated cells, the metals were mainly associated with MT. In summary, Zn-induced MT in rat primary hepatocyte cultures protects against Ag-, Co-, Cu-, Hg-, Ni-, Pb-, and Zn-induced cytotoxicity. This protection appears to be due to the binding of metals to MT with a concomitant reduction of metal content in critical organelles and proteins.