In this paper, we propose to approach the 'biosocial' from a pragmatist angle, by asking how the notions of responsibility and solidarity are at stake in a biosocial world. How does biosociality require us to rethink solidarity and responsibility? How does the concept affect a world full of institutionalized moral and political judgements about responsibility for health and disease? We argue that thinking the biological and the social together implies cultivating new political ontologies with respect to the notions of agency and cause, in ways that severely challenge how they are institutionally mobilized in Western politics and organizations, aimed at what we call the 'imputation' of final responsibilities (e.g. insurance practice and public health policy). We borrow the term 'imputation' from law and jurisprudence to signify a move that identifies a responsible agent, while at the same time defining what it means to be 'responsible' or to act as a 'cause'. We hold that a biosocial world engages its participants into a more difficult and deliberative process of ordering, where the notion of responsibility functions not to settle questions of agency, but as an invitation to rethink both agency and cause in a relational ontology where agents are enabled to respond (response-ability) and become solidary. Taking cue from the pragmatist and speculative works of Donna Haraway, Isabelle Stengers and Bruno Latour, we move away from both nature-nurture as a dichotomy and its annulation as a holism, by placing 'biosociality' under the sign of risky engagements to be made.