Some categories of human beings are particularly vulnerable vis-à-vis medical research. Vulnerability could be considered as the liability to be harmed, exploited, deceived, cheated, wronged, or otherwise unfairly treated, in roughly that descending order of importance. Vulnerable human beings obviously include the incompetent (minors and mentally handicapped adults), the desperately poor, ill or ignorant, prisoners, refugees, pregnant women, subordinates in highly authoritarian systems, etc. Vulnerability in itself does not imply that no research whatsoever should be carried out with such categories of humans but only that it should be carried out only under very special conditions. In this paper I treat of vulnerability in research of particularly developing world populations; of the types of research which exploit such vulnerability, and of why and how research subjects should be protected. The aim in this paper is to stimulate practical reflection on the possible vulnerabilities of potential research subjects that researchers or investigators need to avoid exploiting rather than on an adequate theoretical treatment of the issue.