The synthesis of metallothioneins has been established for Mytilus edulis exposed to cadmium and mercury. We checked if this induction resulted in the synthesis of tissue- or metal-specific isoforms in the gills, the mantle, and the digestive gland that could be used as tool for the characterization of undefined metallic contamination of aquatic ecosystems. An accumulation of metals was observed in the selected organs after 21 days of exposure. The levels of metallothioneins measured by using the polarographic method were significantly increased by cadmium and mercury in the gills (21 days). Size exclusion chromatography showed the presence of a monomer and a dimer of metallothionein of respective apparent molecular weight about 12 kDa and 20 kDa in all samples. They were resolved into five components by anion exchange chromatography in the gills of control or Hg-treated mussels, whereas a sixth isoform was isolated in the gills of cadmium-exposed mussels. In the mantle of mussels exposed or not, five isoforms were separated, and in the digestive gland of mussels exposed or not, six isoforms were separated. The occurrence of a specific cadmium-binding isoform in the gills has to be confirmed in cadmium-contaminated specimens collected in situ before its detection may be used as biomarker of cadmium contamination.