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Using longitudinal data on the entire population of blood donors in an Italian town, we examine how donors respond to a nonlinear award scheme that rewards them with symbolic prizes (medals) when they reach certain donation quotas. Our results indicate that donors significantly increase the frequency of their donations immediately before reaching the thresholds for which the rewards are given, but only if the prizes are publicly announced in the local newspaper and awarded in a public ceremony. The results are robust to several specifications, sample definitions, and controls for observable and unobservable heterogeneity. Our findings indicate that social image concerns are a primary motivator of prosocial behavior and that symbolic prizes are most effective as motivators when they are awarded publicly. We discuss the implications of our findings for policies aimed at incentivizing prosocial behavior.