Approximately 10% of people have O-negative blood. Because it can be transfused into almost anyone, hospitals particularly value such blood. We use this fact, together with the assumption that blood types are exogenously assigned by nature, to design an empirical inquiry into altruism. We also investigate the timing of donations, especially focussing on the behaviour of new and established donors. We show that O-negative blood donors donate no more often than other people. Thus individuals apparently do not exhibit pure altruism. We speculate that instead blood donors may be driven by a broad notion of duty rather than by a far-sighted, rational unselfishness.