A previously published effectiveness study of Project ALERT delivered in schools by outside providers from Cooperative Extension found no positive effects for the adult or teen-assisted delivery of the curriculum despite high-quality implementation. Those findings and the likelihood that more outside providers will deliver evidence-based drug prevention programs in the future, led to this investigation of possible influences of leaders' personal characteristics on ALERT's program effects. Influence of leader characteristics on students' drug use and mediating variables for use were assessed by modeling program effects on within-student change as a function of leader characteristics. Students in classrooms with adult leaders who were more conscientious, sociable, or individuated were more likely to experience beneficial program effects. Students in teen-assisted classrooms with teen leaders who were more sociable or, to a lesser extent, highly individuated, showed more positive effects. Implications for research and practice are discussed.