The effects of the actor-observer relationship (friendship or stranger) were tested to determine the attribution of responsibility for success or failure in a prisoner's dilemma game (PDG). Male subjects (N=80) participated, four subjects per experimental session. Two subjects competed in a non-zero sum, mixed-motive PDG while being observed by two other subjects. Each player was observed by only one observer, either a friend or a stranger. After 20 trials, each player and his observer assigned responsibility for the player's outcome. Observers who were friends to actors assigned more personal responsibility to the actor for success, and less personal responsibility for failure, than did observers who were strangers to actors. Actors who were observed by friends accepted more personal responsibility for success and failure than did actors who were observed by strangers. There are many implications for attribution theory and research. (Author/NRB)