Social inadequacy and lack of social cooperation is one of the defining characteristics of mental deficiency, yet there have been comparatively few investigations into how such children respond to each other. Of even greater significance is the finding that among the reported studies investigating social cooperation between mentally deficient children, there commonly is involved novel apparatus which might in itself have a significant influence upon the cooperative behaviour of the subjects. An experiment was performed to test the hypothesis that use of a novel apparatus will significantly enhance the effects of positive reinforcement of cooperative behavior between two autistic children. Reinforcement alone was found to increase cooperative behavior and reduce uncooperative behavior to a significant degree. These effects were maintained (up to an optimal fixed-ratio schedule), but not significantly increased, when reinforcement and apparatus were used simultaneously. The use of apparatus without reinforcement was found to have no significant effect; in fact, there was some indication that the apparatus might have negative effects on cooperative behavior. It was concluded that use of the apparatus was, at best, irrelevant to the establishment of cooperative behavior. (Author)