Differential treatment of patients by health workers in African medical institutions is acknowledged by patients, health workers and policy-makers alike to be an obstacle in the realization of government objectives of equity in health care. This qualitative study understands the production and legitimization of differential treatment from the perspective of health workers. On the basis of qualitative field material from a hospital in Northern Ghana the relation between socio-cultural, biomedical and bureaucratic aspects of hospital practice is explored through a focus on categorizations of patients. It is concluded that to blame the "bad attitudes" of health workers for differential treatment is not an adequate explanation. It is important to acknowledge that differential treatment can be understood as a form of agency, and is related to the conditions of hospital work and to the professional and social identities of health workers.