Reciprocal altruism may be a useful tool in increasing intentions to join the organ donor register. Further evaluation is required to determine whether this increase in intention can be translated into organ donation behaviour. Statement of contribution What was already known? Demand for organs in the United Kingdom far outstrips supply, so finding strategies to increase registration on the organ donor register could save hundreds of lives per year. Despite the majority of people in the United Kingdom agreeing that organ donation is a good thing, most people do not register as donors. A limited amount of evidence of the impact of perceived reciprocity suggests that encouraging people to consider themselves as recipients and priming ideas of shared responsibility may increase the likelihood of their subsequent willingness to register. What does this study add? Novel evidence that employing a simple reciprocity prime increases organ donor registration intentions. Replication of findings across two separate studies. Novel examination of the impact of mode of delivery of messages to encourage organ donation. A basis for further research into the translation of intentions into organ donor registration behaviour.