Previous research has proposed that tests enhance retention more than do restudy opportunities because they promote the effectiveness of mediating information--that is, a word or concept that links a cue to a target (Pyc & Rawson, 2010). Although testing has been shown to promote retention of mediating information that participants were asked to generate, it is unknown what type of mediators are spontaneously activated during testing and how these contribute to later retention. In the current study, participants learned cue-target pairs through testing (e.g., "Mother:" _____) or restudying (e.g., "Mother: Child") and were later tested on these items in addition to a never-before-presented item that was strongly associated with the cue (e.g., "Father")--that is, the "semantic mediator". Compared with participants who learned the items through restudying, those who learned the items through testing exhibited higher false alarm rates to semantic mediators on a final recognition test (Experiment 1) and were also more likely to recall the correct target from the semantic mediator on a final cued recall test (Experiment 2). These results support the mediator effectiveness hypothesis and demonstrate that semantically related information may be 1 type of natural mediator that is activated during testing. (Contains 2 tables and 1 figure.)