This article explores the development of a novel biomedical research organism, and its potential to remake the species and spaces of translational medicine. The humanized mouse is a complex experimental object in which mice, rendered immunodeficient through genetic alteration, are engrafted with human stem cells in the hope of reconstituting a human immune system for biomedical research and drug testing. These chimeric organisms have yet to garner the same commentary from social scientists as other human–animal hybrid forms. Yet, they are rapidly being positioned as central to translational medicine in immunological research and pharmaceutical development. This article explores the complex relations between species and spaces they seek to enact. Humanizing mice simultaneously moves these animal forms towards the intimate geographies of corporeal equivalence with humans and the expansive geographies of translational research. These multiple trajectories are achieved by the way humanized mice function as both uncertain ‘epistemic things’ and as expansive ‘collaborative things’, articulating mouse genetics with other research, notably stem cell science. In the context of post-genomics, their indeterminacy is critical to their collaborative value; their expansive potential follows as much from their biological openness as from specific expectations. Yet, these new research organisms have both accumulative and disruptive capacities, for there are patterns of interference between these trajectories, remaking boundaries between experimental practices and clinical contexts.