This paper addresses volunteer labor markets where the lack of price signals, non-pecuniary motivations to supply labor, and limited fungibility of supply lead to market failure. To address the causes of the market failure, we conduct a field experiment with volunteer whole blood donors where we introduce a market-clearing mechanism (henceforth: the Registry). Our intention-to-treat estimates suggest that subjects invited to the Registry, regardless of joining, are 66 percent more responsive to critical shortage appeals than control subjects. While the registry increases supply during a critical shortage episode, it does not increase supply when there is no shortage; thus the Registry significantly improves coordination between volunteer donors and collection centers, thereby improving market outcomes. We find evidence that the Registry’s effectiveness stems from crowding-in volunteers with purely altruistic motives and volunteers with a preference for commitment.