This article analyses attempts to enact complexity in postgenomic experimentations using the case of epigenetic research on biomarkers of psychosocial stress. Enacting complexity in this research means dissecting multiple so-called biosocial processes of health differentiation in the face of stressful experiences. To characterize enactments of biosocial complexity, the article develops the concepts of complexity work and complexification. The former emphasizes the social, technical, and material work that goes into the production of mixed biological and social representations of stress in epigenetics. The latter underlines how complexity can be assembled differently across distinct configurations of experimental work. Specifically, complexification can be defined as producing, stabilizing, and normalizing novel experimental systems that are supposed to improve techno-scientific enactments of complexity. In the case of epigenetics, complexification entails a reconfiguration of postgenomic experimental systems in ways that some actors deem ‘better’ at enacting health as a biosocial process. This study of complexity work and complexification shows that biosocial complexity is hardly a univocal enterprise in epigenetics. Consequently, the article calls for abandoning analysis of these research practices using clear-cut dichotomies of reductionism vs. holism, as well as simplicity vs. complexity. More broadly, the article suggests the relevance of a sociology of complexification for STS approaches to complexity in scientific practices. Complementing the existing focus on complexity as instrumental rhetoric in contemporary sciences, complexification directs analytical attention to the pragmatic opportunities that alternative (biosocial) complexities offer to collective, societal, and political thinking about science in society.