Induction support for new teachers is widespread, particularly in the form of mentoring, but research evidence of effectiveness is limited. The majority of existing research has focused on the impact of induction on teacher retention. Of greater interest is the potential impact on student achievement, and on which forms of support are the most effective. One frequently encountered option is between full-release or site-based mentors. This study examines these two mentoring options employed in one large urban district. While mentors received the same training, they differed in case load and release time. A comparison of student achievement gains for classes taught by fourth and fifth grade new teachers, some of whom were supported by full release mentors and some by site based mentors, showed greater gains for classes of teachers in the full-release group, even though the demographic characteristics of the students would have led to the opposite prediction. (Contains 2 tables and 8 figures.)