In each of three experiments, each subject in each condition received five successive short-term memory (STM) tests with phonemically similar consonant trigrams as the items to be remembered. The retention interval for tests 1–4 of all conditions in the three experiments was held constant at 16·5 sec. On test 5 the retention interval was varied across conditions with the three experiments exploring retention intervals in the range 1–21·5 sec. In all three experiments little, if any, forgetting was observed on test 1; in all three experiments proactive interference (PI) was observed to develop across tests. Analysis of the test 5 data indicated that the PI effect of prior test items upon the test 5 target item reached a maximum by 6·5 sec., with no further forgetting beyond this interval. The test 5 data also suggested a reminiscence effect at the longest retention interval of 21·5 sec. The implications of the data for a theory of PI in STM are discussed.