The purpose of this study was to explore how avatar appearance and player-avatar interaction impact prosocial and antisocial online play in World of Warcraft (WoW). The Proteus effect, a phenomenon where players shift their in-game behavior based on the appearance of their avatars, is well documented in experimental settings. Variable player-avatar interaction (PAX) is a newer concept that seeks to capture the way a player relates to his or her avatar as if it were an extension of self, a mere plaything, or a distinct social other. Analysis of survey data from 1,353 WoW players provided partial support for many hypotheses. Significant correlations were present between self-report measures of play motivations (relationship, socializing, teamwork, and competing motivations) and numerical data representing avatars' in-game actions (healing-to-damage ratio and player-versus-player kill ratio). Overall, Proteus effect cues such as avatar attractiveness, gender, size, and clothing color had modest yet significant impact on specific measures of prosocial and antisocial play. Regression models showed PAX dimensions as significant predictors of prosocial and antisocial play. This study contributed to the field of game studies in three ways. First, it established correlations between player motivation and concrete measures of in-game behavior. Second, it tested the influence of the Proteus effect in a large-scale naturalistic setting. Third, it examined the influence of PAX variations alongside Proteus effect cues as predictors of prosocial and antisocial play.