  I estimate the semi-elasticity of blood donations with respect to a monetary
benefit, namely the waiver of user fees when using the National Health Service,
in Portugal. Using within-county variation over time in the value of the
benefitI estimate both the unconditional elasticity, which captures overall
response of the market, and the conditional elasticity, which holds constant
the number of blood drives. This amounts to fixing a measure of the cost of
donation to the blood donor. I instrument for the number of blood drives, which
is endogenous, using a variable based on the number of weekend days and the
proportion of blood drives on weekends. A one euro increase in the subsidy
leads 1.8% more donations per 10000 inhabitants, conditional on the number of
blood drives. The unconditional effect is smaller. The benefit does not attract
new donors, instead it fosters repeated donation. Furthermore, the
discontinuation of the benefit lead to a predicted decrease in donations of
around 18%, on average. However, I show that blood drives have the potential to
effectively substitute monetary incentives in solving market imbalances.
