This paper discusses some of the ethical challenges raised by advanced clinical trials designed to assess the safety and efficacy of vaginal microbicides in protecting women from HIV infection. The ethical principles that guide clinical research involving human subjects require that all participants in such trials be provided available measures known to reduce the risk of HIV infection. However, this will reduce the ability of the study to assess the protective effect of the test microbicide. In addition, providing extensive services to trial participants may be construed as an undue inducement if the study is being conducted among vulnerable groups such as sex workers or women from disadvantaged communities. Suggestions are provided to resolve this dilemma in the planning and implementation of HIV prevention services for trial participants.