The number and scope of Organisation for Economic Cooperation and Development (OECD) in vitro test guidelines (TGs) are increasing in an effort to both improve human relevance and replace in vivo animal testing. In vitro test methods being developed for TG use are increasing the use of human based reagents in combination with or replacing animal derived reagents, and demand for human reagents is likely to grow in the near future. There are a range of issues associated with the ethical use of human reagents, particularly human serum, in the adaptation and development of in vitro TGs, especially to ensure that there is no human exploitation, legal requirements are adhered to, and that the origin of the reagent is assured. To address these concerns, the OECD has instigated a workshop on ethics, sources, availability, and traceability of human based reagents for TG purposes, to be held in March 2019. The focus is to provide guidance on acceptable sources of human serum for use in in vitro TGs, in terms of donor ethics and informed consent regarding commercial use, and quality control for safety and consistent performance, with a view to providing guidance to support the adaptation and/or development of in vitro TGs using human reagents, and to ensure that in reporting the test results to regulators, clearly defined ethical and traceability aspects are adequately addressed, for the Mutual Acceptance of Data principle to be accepted in all OECD member countries. This thought-starter provides a discussion basis to achieve those objectives.