Building domain knowledge is essential to a student's success in any course. Chemistry, similar to other STEM disciplines, has a strong cumulative element (i.e., topic areas continuously build upon prior coursework). We employed the testing effect, in the form of post-exam retrieval quizzes, as a way to improve students' understanding of chemistry over an entire semester. Students (n = 146) enrolled in Introduction to Chemistry were presented with retrieval quizzes released one week after each during-term exam (that covered that exam's content). We measured students' level of quiz participation, during-term exam scores (a control variable), and cumulative final exam scores to determine the effectiveness of implementing a post-exam retrieval quiz system. Most critically, students completing more than 50% of the retrieval quizzes performed significantly better (i.e., more than a half letter grade) on the cumulative final exam than those who were below 50% participation as determined by one-way between-subjects ANOVA and planned follow-up analyses. We found no significant differences between the participating groups on during-term exam scores, suggesting that high achieving students were not more likely than struggling students to participate in the practice testing (and thus benefit from it).