In France, participation in vaccine trials is necessarily a personal act that is not intended to be made public. It does not involve any direct act of solidarity nor does it require any protest or demanding behavior. On the contrary, it requires unconditional compliance with the research protocols. However, it is a form of individual commitment, which is part of a range of social motives that determine the various forms of voluntary involvement in the fight against AIDS: financial donations, voluntary work in associ-ations, political activism, etc. In this study, we use the application letters of the people who responded to the annual recruitment campaigns launched by the Agence Nationale de Recherche sur le Sida et les Hépatites (ANRS) to take part as volunteers in vaccine trials related to the fight against HIV. At the end of a selection process, a certain number of people were recruited every year between 1991 and 2001 and included in a “volunteer network” [The application materials were anonymized by the ANRS before they were sent to us. The network is defined as a group of volunteers (based on the model of blood and bone marrow donors) who have direct contact with the researchers in the form of a partnership and not the classic relationship of therapeutic dependence, are aware of the absence of any direct benefit and of the risks involved, are informed regularly about changes in research, and have the possibility to get to know each other during annual meetings and to share their experiences, hopes, and any disappointments. Selection was performed in a general manner for the purpose of inclusion in the “network” and not for a particular trial. The uniqueness of the network facilitates the distribution of volunteers between trials for the varying needs of the protocols while reinforcing a sense of integration intended to maintain their motivation.]. In addition to the application materials, we add a series of informational interviews with several people in charge of recruitment for the decade studied, as well as about a dozen biographical interviews with volunteers included in the network [The data were made available as part of a research project of the ANRS (2003), transformations des logiques de l’engagement dans la lutte contre le sida: motifs, facteurs et conséquences biographiques de l’engagement dans les essais vaccinaux, (Transformations in the basis of commitment in the fight against AIDS: reasons, factors, and biographical consequences of engagement in vaccine trials), in which Philippe Blanchard and Anne Marijnen collaborated for some of the statistics, informational nterviews with recruiters, and life histories of a group of volunteers. Véronique Doré and Yves Souteyrand, through their efforts, were able to remove all obstacles to the collection and processing of information. For this, we thank them warmly. This article was taken from a lecture given during a study day on June 12, 2015, in cooperation with the Vaccine Research Institute, entitled Du recrutement pour à l’engagement dans la recherche: Interactions et médiations constitutives des recherches en prévention contre le VIH au Nord (Recruitment for engagement in research: interactions and mediations in research on the prevention of HIV in Western Countries). It also benefited from the comments of participants.]. The research consisted of an exploration of 2 connected aspects: the rationale behind voluntary engagement (the act of submitting an application) and the rationale underlying the recruitment (the work involved in the selection process of people by the ANRS). In most studies, the determinants of engagement are identified based on an analysis of the attributes of the selected volunteers but without showing whether these attributes distinguish them from applicants who were not selected. In addition, “motivations” for engagement are usually collected a posteriori. In this case, the corpus of initial application letters offers the possibility of comparing one population of applicants with the people who were eventually included in the network and distinguishing those who dropped out or refused to be included.