It is widely agreed that living kidney donation is permitted but living kidney sales are not. Call this the Received View. One way to support the Received View is to appeal to a particular understanding of the conditions under which living kidney transplantation is permissible. It is often claimed that donors must act altruistically, without the expectation of payment and for the sake of another. Call this the Altruism Requirement. On the conventional interpretation, the Altruism Requirement is a moral fact. It states a legitimate constraint on permissible transplantation and is accepted on the basis of cogent argument. The present paper offers an alternative interpretation. I suggest the Altruism Requirement is a moral fiction-a kind of motivated falsehood. It is false that transplantation requires altruism. But the Requirement serves a purpose. Accepting it allows kidney donation but not kidney sale. It, in short, rationalizes the Received View.