Although numerous studies have documented the effects of microplastics (MP) exposure on marine organisms, there is still uncertainty regarding to what degree such effects are caused by physical toxicity from particles or chemical toxicity from leaching chemicals associated with MP. In this study, we investigate the contribution of these two types of effects through a sublethal exposure study on the benthic polychaete deposit feeder Capitella spp. The study is conducted with MP produced from several materials, in order to see how effects might vary as a function of polymer type and additive chemical content. Firstly, solvent extracts from 50 plastic consumer products will be screened using the Bacteria Luminescence Toxicity (BLT) test to establish the baseline toxicity induced by the additive chemicals present. Based on this, a sub-group of materials representing both high and low chemical toxicity will be selected. The selected material will be cryomilled into irregular, polydisperse MP. Capitella spp. will be exposed for a full life cycle (appr. 21 d) to four treatments: MP with associated chemicals, the chemical leachate from the same material, c̈leanM̈P with the leachate removed and a control with natural silica particles. For this ongoing work, we hypothesise (i) that exposure to MP with high levels of associated chemicals and MP leachates derived from those materials will affect sublethal parameters of Capitella spp. to a higher degree than the c̈leanM̈P, and (ii) that c̈leanM̈P will have the same effect as the natural particles. The study also aims to assess with the BLT screening test is suitable for predicting toxicity in higher trophic levels.