International bureaucracies can expand their activities into new domains, even when these are remote from their formal mandates. Asking how they do this, this article reveals that expansion often goes unnoticed, because international secretariats typically start new activities in a way which is informal, mundane and depoliticized. Through an examination of UNESCO's and WHO's expansion into the domain of bioethics, the article argues that international bureaucratic tactics revert to a three-fold strategy mixing technicalization, expertization and naturalization. This suggests that international bureaucracies' autonomy does not depend on their fixed characteristics, as defined by their legal-institutional mandates, capacities, or level of expertise. Expansion takes place, rather, when such resources are efficiently activated, assembled, or developed, thus shedding light on the need to examine what international bureaucracies 'do', rather than what they 'are', in order to capture their influence in global governance.