Ss read either affirmations or denials of target propositions that ostensibly came from either newspapers or reference volumes. Denials of the validity of a proposition that was already assumed to be false increased Ss' beliefs in this proposition. The effect generalized to beliefs in related propositions that could be used to support the target's validity. When denials came from a newspaper, their "boomerang effect" was nearly equal in magnitude to the direct effect of affirming the target proposition's validity. When Ss were asked explicitly to consider the implications of the assertions, however, the impact of denials was eliminated. Affirmations of a target proposition that was already assumed to be true also had a boomerang effect. Results have implications for the effects of both semantic and pragmatic processing of assertions on belief change.