At the end of 2021 and the beginning of 2022, we held two expert working group meetings on refining studies involving bone marrow ablation and reconstitution in mice. These procedures are used in different fields of research, including studies of immune system function, ageing and cancer biology, and can induce ‘severe’ suffering in the animals used. The working group identified key welfare issues and set out practical refinements to reduce suffering and improve welfare. Our report is free to download and includes a flow chart to help you implement practical refinements at every stage.