Spillovers of prosocial motivation can enhance the provision of public goods, and have implications for the cost-benefit analysis of policy interventions. We analyze a large-scale intervention among dyads of pre- registered blood donors. A quasi-random phone call provides the instrument for identifying endogenous and exogenous social interaction. The phone call has a strong effect on the recipient's propensity to donate. Between 40% and 44% of that behavioral impulse is transmitted within dyads due to motivational spillovers. This creates a significant social multiplier to policy interventions, with estimates ranging between 1.6 and 1.8. There is no evidence for exogenous social interaction.