The effect of social reinforcement delivered to target subjects on the attentive behavior of adjacent peers was examined in a classroom setting. In a combined reversal and multiple baseline design, two pairs of mentally retarded children were sequentially exposed to three reinforcement phases. After baseline rates of attentive behavior were obtained, praise was delivered to the target subject in each subject pair for attentive behavior. After a reversal phase, praise was delivered contingently to target subjects for inattentive behavior. In a final phase, contingent praise for attentive behavior was reinstated for the target subjects. Throughout the study, nontarget subjects received no direct reinforcers. The results indicated a vicarious reinforcement effect. Reinforcing attentive behavior of target subjects increased this behavior in adjacent peers. However, reinforcing inattentive behavior of target subjects also increased the attentive behavior of adjacent peers. The effects obtained through vicarious reinforcement were considered to reflect the discriminative stimulus properties of reinforcement, which may serve as a cue for the performance of nonreinforced peers.