The antisocial boys ranged in age from 8 through 16 years and were referred to the program because of frequent fights with other youths or adults, uncontrollable tantrums, truancy, theft, and other antisocial acts. Treatment proceeded while nearly all of the subjects lived in the open community and engaged in group activities with other boys not identified as having behavior problems. The 3-year study found that the mixed groups produced highly favorable outcomes for referred youths and had few, if any, adverse consequences for the nonreferred youths. Experienced group leaders were much more effective than inexperienced ones, and peers played an influential role in attaining treatment outcomes. Program gains are likely to be transferable to the participants' natural environments. Results support intervention programs directed at antisocial behaviors which are not serious but which occur frequently; they also show the usefulness of programs that engage antisocial boys with conventional youths while weakening their ties with delinquent peers. Prosocial modeling and peer reinforcement for conventional behavior can shape the behavior of antisocial youths. Experienced group workers can generate favorable outcomes for referred youths, while mitigating adverse outcomes for nonreferred youths. Figures, tables, chapter notes, an author and subject index, and about 300 references are provided.