The current study is the first to investigate neural correlates of infants’ detection of pro- and antisocial agents. Differences in ERP component P400 over posterior temporal areas were found during six-month-olds’ observation of helping and hindering agents (Experiment 1), but not during observation of identically moving agents that did not help or hinder (Experiment 2). The results demonstrate that the P400 component indexes activation of infants’ memories of previously perceived interactions between social agents. This leads to suggest that similar processes might be involved in infants’ processing of pro- and antisocial agents and other social perception processes (encoding gaze direction, goal directed grasping and pointing).