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During an infant vaccination trial in the Niger, difficulties emerged as parents began to withdraw their children. In this article, the authors seek to analyse the reasons for such a change of attitude on the part of parents. Great care was also taken to ensure that recommendations on ethics, including those regarding to developing countries, were scrupulously respected. Three main reasons were given for refusal: fear of blood sampling, due to the risk of exploitation, negative perceptions of experimentation due to an apparent ambiguity with regard to its presentation, and the need to give formal consent, a procedure considered contrary to local practice. In particular, the article discusses the difficulties relating to collective consent, as proposed for developing countries, and the constraints relating to individual consent in traditional societies. It also underscores the reasons for combining both approaches and the importance of respecting local customs.