In this paper, I explore the ethics of subject selection in the context of biomedical research. I reject a key principle of what I shall refer to as the standard view. According to this principle, investigators should select participants so as to minimise aggregate risk to participants and maximise aggregate benefits to participants and society. On this view, investigators should exclude prospective participants who are more susceptible to risk than other prospective participants. I argue instead that investigators should select subjects in accordance with an alternative principle: formal equality of opportunity. According to this principle, investigators must treat all prospective participants the same unless differential treatment is warranted by the scientific goals of the study or the need to promote participants' medically related interests. All prospective participants (1) who meet the scientifically defined eligibility criteria and (2) for whom participation is consistent with their medically related interests should have an equal, formal opportunity to participate in the study. Prospective participants should not be excluded simply because they are more susceptible to risk than others.