The clinical trial is the major investigational tool of clinical medicine. Two recent reports highlight the fact that the most often quoted mechanisms for the protection of research subjects, viz., research ethics board review and eligibility criteria, are insufficient to achieve this end. In this paper, we argue that the prime mechanism for the protection of persons in clinical trials should be the clinical judgement of the physician-investigator. The clinical investigator has a duty to protect subjects from both harm and undue risk. It is argued that the clinical investigator has a duty to screen for, and exclude, potential research subjects who may be unduly vulnerable to the risks of a particular clinical trial. In order to fulfill this obligation, the investigator should personally screen each potential research subject at the time of accrual. In larger trials in which this is not feasible, this task could be delegated to another appropriately qualified health care professional, with the principal investigator retaining personal responsibility. To reinforce and make explicit this legal and moral duty, we propose that the investigator sign a statement, appended to each subject's consent form, to attest that this duty has been responsibly discharged.