This paper examines the origins of aseptic surgery in the German-speaking countries. It interprets asepsis as the outcome of a mutual realignment of surgery and laboratory science. In that process, phenomena of surgical reality were being modelled and simplified in the bacteriological laboratory so that they could be subjected to control by the researcher’s hands and eyes. Once control was achieved, it was being extended to surgical practice by recreating the relevant features of the controlled laboratory environment in the surgical work place. This strategy can be seen in the adoption of Robert Koch’s bacteriology by German-speaking surgeons, and the resulting technical changes of surgery, leading to a set of beliefs and practices, which eventually came to be called ‘asepsis’.