Human embryonic stem cells are elusive, recalcitrant entities that resist characterization and standardization. Without agreements about what the cells are and how best to systematize cell culture and testing, data cannot be extracted meaningfully, the nascent field will be slow to stabilize, and significantly, there may be safety risks for patients. I discuss efforts to characterize cells definitively and standardize practices across uniquely derived lines, labs, and researchers. I argue that such efforts are made more complicated by layered identities imposed on them by classification conventions, interactions with researchers and laboratory environments, and inheritances from genetic ancestry. The need to understand and possibly capitalize on such distinct, cumulative identities is in tension with the desire to stabilize the field under conditions of political and scientific uncertainty. The article links STS work on standardization with anthropological perspectives on identity and material culture in science.