Estuarine sediments frequently are repositories and therefore potential sources of anthropogenic contaminants. Many organic and metallic chemical compounds released into aquatic systems bind to particulates and so accumulate in the sediments, thus, sediments become repositories of contaminants in estuaries. These may also cause contamination through diffusion of porewater, resuspension of particulates and dispersal of benthic fauna. There is a need to assess the biological affects of these anthropogenic contaminants because they may be toxic to infauna and bottomfish. Sediment toxicity bioassays are a means for carrying out such an assessment and primarily provide data on toxicity by measuring the effects on the test organism. Existing sediment toxicity bioassays rely on a battery of aquatic toxicity tests, which are based on the extraction of pore water, and elutriate from sediments and then subjecting these sediment phases to toxicity testing regimes. Two estuarine bivalve molluscs, Scrobicularia plana and Tapes Semidecussatus were used to assess the ecotoxicity of field-collected sediments from estuarine and coastal areas around the Irish and English Coast over a 3-year study period. A variety of endpoints were measured during the study including survival in air, behaviour, animal condition, biochemistry, soft tissue metal concentrations, lysosomal membrane integrity and histopathology. Of these endpoints, the most sensitive were survival, survival in air, lysosomal membrane integrity, behaviour and histopathology.