This work is an assessment of, and a contribution to, the development of a sociology of medical knowledge - including the construction of medical opinion, the fabric of medical discourse and the medical construction of the body. Extensive research on the work of haematologists is used to demonstrate the strengths and weaknesses of the existing understanding of medical knowledge. Topics covered include: the place of interaction among doctors, rather than between doctors and patients, in defining the construction of medical knowledge; the ways in which clinical opinion is socially produced and the nature of the local settings in which this process occurs; and the relations between medical knowledge, medical language, and the increasingly technological contexts of contemporary medical practice.