Sub-lethal exposure of the marine amphipod Gammarus locusta to a range of copper concentrations (Cu) in water and spiked sediments were performed, and the resulting bioaccumulation of Cu into intracellular granules investigated. The presence of Cu-granules was demonstrated histochemically by the rubeanic acid method. The granules were quantified by automated image analyses (expressed as volume fraction, VV). The metal composition of the granules was characterized by X-ray microprobe analysis. The results showed that granules, rich in copper and sulphur, were formed in response to Cu exposure in water and sediment. These granules appeared in the B-cells of the hepatopancreas. VV values increased over the dose-range of Cu compared with control in water (P<0·001) and sediment (P<0·01) exposures. The abundance of the granules also increased with increasing whole-body Cu content, suggesting that at least part of the increasing Cu level was incorporated into the granules, as a strategy for Cu detoxification along with normal storage of Cu during the moult cycle. The presence of sulphur within the granules is thought to represent an organic detoxification mechanism for Cu. The formation of Cu-granules as a cellular response is a useful biomarker of Cu-exposure in ecotoxicological studies with amphipods. The rubeanic acid method is a useful screening tool for this copper.