Not enough kidneys are donated each year to satisfy the demand from patients who need them. Strong moral and legal norms interfere with market-based solutions. To improve the supply of kidneys without violating these norms, we propose legal reforms that would strengthen the incentive to donate based on altruistic motives. We propose that donors be permitted to donate kidneys in exchange for commitments by recipients or their benefactors to engage in charitable activity or to donate funds to charities chosen by donors. And we propose that charities be permitted to create Altruism Exchanges, which would permit large numbers of altruists to make charitable exchanges with each other, including but not limited to kidney donations. Altruism Exchanges would solve two significant problems with the current system of voluntary kidney donations: the risk of default and the lack of liquidity.