MPs' uptake and tissue accumulation were investigated in mussel exposed to a single dose (2.85 mg ind, 3 mg l) of a heterogeneous mixture of irregularly shaped particles of HDPE (mainly ≤10 μm), followed by a 7 days depuration period. The results showed that mussels efficiently cleared MPs from water during exposure, and that MPs were accumulated in digestive gland and gills during depuration. In digestive gland, the amount and size of the MPs accumulated decreased with time, indicating a slower processing and elimination of small MPs than of larger ones. In gills, MPs' burdens increased with time, the MPs accumulated were the smallest ones, suggesting the translocation of small MPs from the digestive system to the gills. The hazardous potential of the smaller fraction of MPs (≤4 μm) underlined that more focus should be directed towards the accumulation and effects of this fraction of MPs in the marine environment.