A historical case study is used to characterize the role of experiment in stem cell biology. I argue that this role is not fully captured by hypothesis-testing in the traditional sense. A pivotal episode of blood stem cell research illustrates another, complementary aspect of experiment, which remains influential in stem cell biology today. Careful examination of this case shows how details of experimental methods, social organization of experimenting communities, and structures of abstract models, combine to yield knowledge of blood stem cells. I conclude by discussing "social experiments" in light of recent accounts of model-theory relations.