Testing can enhance memory, but what is the optimal placement of tests during a learning episode? Retrieval success during retrieval practice has been recognised as an important factor to enhancing long-term memory. The grain size hypothesis therefore proposes that several interim tests of smaller amounts of information interspersed throughout learning should result in better retention than a single test at the end of learning, as retrieval success during practice is expected to be higher. However, previous research has found that although interim tests result in better practice performance than end tests, this does not translate into an advantage at cumulative assessment when using complex materials. We evaluated the grain size hypothesis using lists of related (Experiment 1) and unrelated (Experiment 2) words and found that interim tests enhanced both practice and cumulative assessment performance. However, we still observed considerable forgetting of successfully recalled items in the interim test group in the cumulative test. Experiment 3 therefore investigated whether desirable difficulty during practice is important for long term retention. The results will be discussed in light of theories of retrieval practice.