Two experiments evaluated how testing can support natural concept learning. We used the procedure of Jacoby et al. (2010) to examine how testing can influence generalization of knowledge, and how that knowledge is retained over time. In two experiments, subjects learned to classify birds from different families and were then were tested on the birds they learned plus additional birds from the same families. Memory and generalization were tested after the study phase and again after varying delays. Results from both experiments indicate that testing does indeed enhance generalization in addition to memory, and that the benefits are maintained over the duration of our experiments—up to 25 days. These results indicate that testing is a potent means of enhancing inference, and that the benefits are long-lasting.