What does solidarity do for bioethics?

Bioethical work on solidarity has yielded an array of divergent conceptions. But what do these accounts add to normative bioethics? What is solidarity's distinctive social normative role? Prainsack and Buyx suggest that solidarity be understood as the 'putty' of justice. I argue here that the putty metaphor is deeply insightful and-when spelled out in detail-successfully explicates solidarity's social normative function. Unfortunately, Prainsack and Buyx's own account cannot play this role. I propose instead that the putty metaphor supports a conception of solidarity as equity. This proposal enables us to answer whether and when we should act in solidarity, and with whom, while also capturing the putty metaphor and hence answering a basic question: what is solidarity for?