Testing typically enhances subsequent recall of tested material. In contrast, it has been proposed that consolidated memories can be destabilized when reactivated and then need to be reconsolidated in order to persist. Learning new material immediately after reactivation may disrupt reconsolidation. We investigated whether the well-known benefits of testing are counterbalanced by a tendency for retrieval to make memory more susceptible to interference. Participants learned 20 English-Swahili word pairs (List 1) on Day 1. On Day 2 one group of participants took a reminder test of List 1 immediately before learning 20 English-Finnish word pairs sharing the same cues (List 2). A second group learned List 2 without taking the reminder test of List 1, a third took the reminder test alone, and a fourth group did nothing on Day 2. On Day 3 all participants took a final test. The reminder test, far from impairing List 1 memory, enhanced it, revealing a testing effect. Furthermore, List 2 learning disrupted List 1 memory when there was no reminder test, but reminder testing immunized the memory against interference. (Contains 2 tables and 1 figure.)