This paper addresses the external validity of experiments investigating the characteristics of altruism in the voluntary provision of public goods. We conduct two related experiments that allow us to examine whether individuals who act more altruistically in the context-free environment are also more likely to act altruistically toward a naturally-occurring public good. We find that laboratory behavior can be predictive of contributions toward naturally-occurring goods, but not in a uniform way. In fact, parametric measures of altruism do a poor job of predicting which subjects are most likely to contribute to a naturally-occurring public good.