In this article, I look at performance artist Kira O'Reilly's recent work in biology laboratories. I show howO'Reilly understands her work in the lab as 'play'. This denotes both an attitude oriented towards indeterminacy and improvisation, as well as a (not always 'fun') play of desire and intimacy. ConsideringO'Reilly playing in the lab in these ways allows for investigations of cultural difference between arts and science, and the negotiations of interdisciplinariy. It also opens up questions about the status of 'work' within both disciplinary areas. Finally, it suggests broader philosophical questions about indeterminacy and the possibility of change.While not all of the work I look at in this article is explicitly situated as theatre or performance, this investigation of play is particularly useful for performance studies. I pay attention to O'Reilly's descriptions of interactions in the labs where she worked, which themselves have a complex status as performance; they are time-based and process-led, though they may lack formal documentation or representation. Yet it is within these informal and sometimes unspoken exchanges of knowledge, alongside the more constructed artistic outputs, that O'Reilly's tactics of playfulness are developed. This is of broader importance for the study of performance, and for the use of performance practice within other disciplines. The informal, unspoken, and blurrily defined edges of disciplinarity should sometimes be played with so that different forms of knowledge can emerge. Performance is a particularly compelling place for this type of play.