Describes a nine-month study conducted in an urban middle school to attempt to reduce the level of aggression of a small group of seventh graders by assigning them a positive role as mediators to fifth- and sixth-grade disputants. Pretest and posttest measures of self-concept and teacher's perception of problem behavior showed dramatic improvements. However, impulsivity scores and the rate of disciplinary referrals remained constant. (Contains 35 references and 3 tables.) (GCP)