Using data from a survey of 923 medical interns in Mexico, this article analyzes preferences for type of medical activity (general or specialized practice), type of site (ambulatory or hospital), and type of medical care institution (public assistance, social security, or private). Four independent variables are examined: social origin, medical school, place of internship, and assimilation to the internship hospital. The great majority of the interns expressed a preference for specialty practice, hospitals, and social security institutions. The role of social origin was to selectively direct students into different medical schools. From then on, the structural attributes of the school itself and of the place of internship, as well as the socialization experiences that took place there, emerged as the most important determinants of career preferences. Such a process, however, tended to produce a "social specialization" of interns in terms of the role they expect to play in the medical field. It is argued that this kind of specialization has negative implications for the professional status of physicians, although it also poses a challenge to the development of innovative theories about the process of professionalization in medicine.