Subjects included over 400 boys referred for antisocial behavior and nonreferred boys. Subjects were assigned to a behavioral, traditional, or minimal treatment group led by an experienced or inexperienced therapist. The modes of group composition were referred only, nonreferred only, or mixed. Behavioral data were collected by time sampling by nonparticipant-observers and via questionnaires administered to referral agents, parents, group leaders, and youths. Results show professional training to be an integral determinant of treatment outcomes for both antisocial and nonsocial youth: only experienced therapists generated significant longitudinal gains. A substantial number of youths became more antisocial with inexperienced therapists. Treatment method was not a potent predictor of positive outcome. Interactive analyses suggest that failure of traditional treatment was mediated by peer group composition. The greatest deterioration in behavior was shown by referred boys in unmixed groups treated with traditional methods by an inexperienced therapist. Results point to the effectiveness of integrated treatment programs and demonstrates that the behavior of antisocial boys can be shaped by prosocial modeling and peer reinforcement of conventional behavior. Limited followup data suggest that treatment gains are more likely to generalize and adverse labeling is less likely to occur when treatment takes place in a community agency. Further sources of information and five additional readings are appended.