Internists and general practitioners were interviewed in random sample to determine their opinions and use of 24 screening tests for patients aged 40 to 65 years. Present practice patterns ranged from urinalysis, done by 97% of physicians to mammography, done by 1%. A significant proportion reported many tests ideally desirable, although not now usually done. Most frequently cited reasons for not using ideally desired tests included patients' expense, physicians' reluctance to use tests without clinical indication, lack of office facilities, and amount of physicians' time and inconvenience. Most common reasons for not wanting other tests used were reluctance to use them without clinical indication, low yield, and expense. Although an automated multiphasic screening system may solve problems of logistics and expense, physicians' dependence on clinical indication and fear of losing medical control may reduce utilization.