Spillovers of prosocial motivation are crucial for the formation of social capital. They facilitate interactions among individuals and create social multipliers that amplify the effects of policy interventions. We conducted a large-scale intervention study among dyads of blood donors to investigate whether social ties lead to motivational spillovers in the decision to donate. The intervention is a randomized phone call making donors aware of a current shortage of their blood type and serving us as an instrument for identifying motivational spillovers. About 40% of a donor's motivation spills over to the other donor, creating a significant social multiplier of 1.78.