In the vast majority of experiments documenting the existence of reciprocity subjects are endowed with windfall funds. In some situations such endowments might create a so-called "house money effect". We identify two reasons why the source of endowment might matter for negative reciprocity: (1) Using earned ? as opposed to windfall money ? might increase the costs of negative reciprocity due to this money being in a different mental account and thus lead to less retaliation. (2) Decreasing a decision-maker?s endowment consisting of earned money might be considered a stronger violation of property rights and lead to more retaliation. We test our conjectures in an experiment and find that subjects retaliate more in both cases.