The extent of autometallographical black silver deposits (BSD) has been semiquantified at the light microscope in the gills and digestive gland of either control mussels or Zn-polluted mussels after depuration and on exposure to sublethal concentrations of Cu, Zn, and Cd. The BSD extent in the gills and digestive gland of control mussels was much reduced compared to that in other experimental mussels. The extent of BSD in the gills of depurating mussels was reduced at short depuration times due to decreased levels in the abfrontal cells while in the digestive gland it did not change with the depuration period. The extent of BSD in digestive lysosomes of Cu- and Zn-exposed mussels followed a logarithmic pattern in relation to metal concentration increasing with metal concentrations in the digestive gland. However, a reduced extent of BSD was related to the presence of high metal concentrations under Cd-exposure conditions. This is because the great extent of BSD present in the lumen of the digestive tubules was not taken into account to carry out semiquantification, but, however, the chemical analysis measured the Cd content of these BSD. As such, the extent of BSD in digestive lysosomes followed a logarithmic pattern with total metal concentrations in the digestive gland of Cd-exposed mussels. Therefore, the semiquantitative estimation of BSD in the digestive lysosomes could be considered a reliable index to reflect changes in metal bioavailability in sea water.