Medical screening in the workplace was discussed. Medical screening is commonly practiced in industry; 34 percent of those working in the United States have a periodic health examination. The National Institute for Occupational Safety and Health (NIOSH) often recommends periodic and preplacement screening to Occupational Safety and Health Administration (OSHA) as part of proposed health standards. Screening requirements have been promulgated by OSHA for 24 substances, are recommended by NIOSH in 100 criteria documents, and have been jointly recommended by OSHA and NIOSH for 386 chemicals. It is noted that at present no set of principles adequately guides the development of screening recommendations in the workplace, or evaluates the adequacy of existing recommendations. A set of principles appropriate for medical screening in the workplace was proposed. These include screening for diseases deemed important to the individual or to the community, screening for recognized diseases for which an accepted treatment exists, planning for the care of participating workers who demonstrate abnormal screening results, detecting diseases when they are in the latent or early symptomatic stage, obtaining suitable qualified personnel for administering screening tests, and analyzing screening results to assess the adequacy of such primary preventive efforts as engineering controls and biological monitoring. The authors conclude that differences between the modified set of principles and previous principles mainly concern characteristics of the screening test and the possibility of using screening to improve efforts in primary prevention.