Subcellular Cu sequestration was examined in the digestive gland, kidney and gill of the Antarctic bivalve Laternula elliptica collected from a Cu-elevated bay in King George Island. Cu was associated with both the soluble cytosolic and insoluble particulate cell fractions in all three organs, but their relative contributions to Cu sequestration varied with tissue type and the total amount of Cu accumulated. Low-molecular-weight (10-13 kDa) metallothionein-like proteins were the major Cu-binding ligands in the cytosol of all three organs. Significant portions of the cytosolic Cu were also bound to proteins with different molecular weights in the kidney and gill. A strong immunological response to a metallothionein (MT) antibody confirmed the presence of MTs in all three organs. Numerous electron-dense granules, which are likely to be metal-rich, were observed in renal epithelial cells by transmission electron microscopy, suggesting that these granules also play a role in Cu sequestration.