The Organisation for Economic Co-operation and Development (OECD) has published a September 2025 document entitled Guidance Document on Assessing the Apparent Accumulation Potential of Nanomaterials. The document is intended to provide specific advice on how to conduct bioaccumulation tests in fish with nanomaterials following Test Guideline (TG) 305 (OECD 2012), once it has been decided that such a fish test is needed. The purpose is to offer guidance on the critical points indicated in the Introduction section to generate harmonized results that can inform reliably on the accumulation potential of nanomaterials. Considerations about the most appropriate testing regime (i.e., aqueous versus dietary), guidance for the preparation of stable and reproducible suspensions, indications on the appropriateness of the approaches proposed to derive the bioconcentration factor (BCF) and the biomagnification factor (BMF), the validity of BMF to assess the bioaccumulation of nanomaterials, and the availability of methods to determine tissue/fish concentrations are included in the document.