Previous research suggests that the actions of a confederate can alter participants' gambling behavior. In the present experiment, male participants played Blackjack either alone or in the presence of a confederate. The confederate either quit early in the session or played for the entire session. Across sessions in which the confederate played for the entire session, how much the confederate bet per hand and how accurately he played were manipulated. Participants gambled significantly more money across the session when a confederate played the entire session than when a confederate left early in the session. During sessions in which the confederate played for the entire session, the number of hands participants played and their total amount bet varied directly with the confederate's bet size. Overall, the results indicate that gambling behavior can be indirectly influenced by another's gambling behavior, which may provide useful data for researchers and therapists seeking to predict and control gambling behavior. (Contains 1 table and 2 figures.)