Medical screening and biological monitoring are two tech-niquesin a continuum of practices used to prevent occupational disease. Medical screening is the examination of workers for pathological conditions. Biological monitoring is the examination of the worker for the presence of an intoxicant or its metabolite. These techniques are common in industry, and are frequently components of health standards recommended by the National Institute for Occupational Safety and Health (NIOSH) to the Occupational Safety and Health Administration. However, an underlying set of principles for medical screening and biological monitoring in the workplace has not been delineated. In this paper, a modification of the principles for screening in the community is proposed that will be more appropriate to the workplace. This modified set of principles is not a statement of the policy of NIOSH on this important subject, but rather a presentation of the authors' views as a starting point for dialogue on these issues.