The accumulation of Cd and Pb in the gills of the lamellibranch mollusc Mytilus edulis has been studied by electron microscopy, X-ray microanalysis, atomic absorption spectroscopy and radionuclide monitoring. The patterns of accumulation of the two elements differ markedly as do the sites of deposition whithin the gills. Lead is found extracellularly as crystalline deposits in the basal lamina which forms the capillary walls of the gill lamellae. The Pb is found associated with Ca in equiatomic ratios and occurs either as a mixed or complex carbonate. Cadmium is always associated with S and frequently with P in membrane bound vesicles within the cells of the gill epithelium and in the amoebocytes. The S is probably attributable to the presence of cysteine residues in a metal binding protein which can be extracted from the gills. Analysis of the metal binding protein shows that it binds Ag, Cd, Cu, Fe, Hg, Sn and Zn. Its amino acid composition is similar to that reported for eels and limpets but has a lower cysteine content than mammalian metal binding protein.