This document presents the final report of a 4-year project (ending in 1991) which conducted six studies of the impact of various peer-mediated interventions on the acquisition, generalization, and maintenance of social interaction skills of preschoolers with severe disabilities. The report covers project objectives, the project's theoretical framework, descriptions of the six individual studies, methodological problems, and the project's impact. Major impacts are reported in terms of products, including observational protocols for assessing preschool children's social and supportive interactions, and dissemination activities such as publication of journal articles, presentations at professional conferences, and dissemination of classroom instructional materials. Reports of three of the studies are attached. Titles and authors are: (1) "Promoting Positive and Supportive Interactions between Preschoolers: An Analysis of Group-Oriented Contingencies" (Frank W. Kohler et al.); (2) "Using a Group-Oriented Contingency To Increase Social Interactions between Children with Autism and Their Peers: A Preliminary Analysis of Corollary Supportive Behaviors" (Frank W. Kohler et al.); and (3) "The Overtures of Preschool Social Skill Intervention Agents: Differential Rates, Forms, and Functions" (Frank W. Kohler et al.). The other three studies dealt with the effects of immediate and delayed reinforcement on the cross-setting generalization of children's social interactions, analysis of performance variability in peer-mediated interventions for children's social interactions, and programming for generalization of peer-mediated interventions. The overall report contains 10 references and the individual reports also contain references. (DB)