The results of a two-year study indicate that collaborative testing is a valuable pedagogical strategy that can both assess and enhance student learning. After finishing their first attempt at each exam, students were given a second attempt either working collaboratively in small groups or individually with open books and notes. Collaborative testing consistently produced significantly higher test scores. In addition, students' retention of course content is shown to be improved when collaborative testing is used. Concerns about grade inflation are alleviated through the use of proportionally weighted grading. (Contains 4 tables.)