The effects of uninstructed and teacher instructed cooperative group learning on the interactive behaviors of 32 nonhandicapped and 16 handicapped (moderately mentally retarded) elementary students and also on the attitudes of nonhandicapped students toward handicapped persons were investigated. In the teacher instructed condition, the teacher gave instruction and feedback on cooperation. In the uninstructed condition, teacher discussion and feedback were focused on the task. In both conditions, students engaged in activities designed to promote interdependence and encourage participation of all group members. Social interaction behaviors during group science activities and free play were measured along with attitudes. Among findings were the following: (1) the social interaction behaviors of students during cooperative group activities differed significantly in the teacher instructed and uninstructed treatment conditions; (2) during the teacher instructed free play condition, the handicapped Ss engaged significantly more often in active participation (cooperative play and conversation); and (3) attitude pre- and post-scores did not differ significantly for teacher instructed, uninstructed, or control groups. A nine-page reference list is provided, as well as appendices that include: (1) a science lesson sequence, teacher scripts, and a teacher observation form; (2) observation categories, definitions, and protocols; (3) an attitude scale; and (4) tables of results. (CL)