In this paper, I examine the historical development of a clinical test for `familial adenomatous polyposis' (FAP), an inherited condition which often leads to cancer of the colon. By paying attention to continuities and changes, especially in the engagement between those medical professionals and patients involved in the development of this test, I reconsider the relationship between knowledge and practice, with an eye to Michel Foucault's and Paul Rabinow's competing notions of `bio-power' and `biosociality'. I conclude by offering some speculative suggestions for further avenues of inquiry into the constitution of the `subject'.