This study investigated changes in the adaptive social behavior of educable, mentally retarded adults who participated in a group discussion program in a state hospital and training center. A rating scale was developed for judging behavior changes resulting from 20 group discussions. Three experimental and three control groups used 74 subjects, males and females, aged 18 to 45, within the borderline to moderate range of retardation. Two groups were formed according to community placement experience or lack thereof. A third experimental group was heterogeneous, consisting of participants with and without community placement experience. All experimental groups showed gains on behavior scale scores, especially the group without community placement experience. No significant difference in behavior change was noted between the group with community placement experience and the heterogeneous group. It appears that the selective use of group discussion can do much to maintain and improve the social adaptive behavior of educable, adult mental retardates; and that group composition (preferably homogeneous) is important to the effectiveness of group discussion as a learning procedure. Recommendations included adaptations of this procedure to the diagnosed needs of participants, minimal use of lecture techniques, and avoidance of abstraction. Longitudinal studies and other research were also advised. (Included are 51 references and 37 tables and figures.) (author/ly)