This paper tests the effect of a matching mechanism on donations in a controlled fieldnexperiment. We match the donations of students at the University of Zurich who, each semester,nhave to decide whether they wish to contribute to two Social Funds. Our results support thenhypothesis that a matching mechanism increases contributions to a public good. However, theneffect depends on the extent to which the contributions are matched. Whereas a 25 percentnincrease of a donation does not increase the willingness to contribute, a 50 percent increase doesnhave an effect. In addition, people need to be socially inclined to react to the matchingnmechanism. The field experiment provides some evidence suggesting that the matchingnmechanism crowds-out the intrinsic motivation of giving.