After the failure of three large clinical trials of vaginal microbicides, a Nature editorial stated that the microbicide field "requires a mechanism to help it make rational choices about the best candidates to move through trials" . In this month's debate, James Lavery and colleagues propose a new mechanism, based on stopping trials early for "opportunity costs." They argue that microbicide trial sites could have been saturated with trials of scientifically less advanced products, while newer, and potentially more promising, products were being developed. They propose a mechanism to reallocate resources invested in existing trials of older products that might be better invested in more scientifically advanced products that are awaiting clinical testing. But David Buchanan argues that the early stopping of trials for such opportunity costs would face insurmountable practical barriers, and would risk causing harm to the participants in the trial that was stopped.