The following condition is required for tit-for-tat to be the best strategy in repeated prisoners' dilemma games: (a) there must be a reasonably stable set of players, preferably two, (b) each firm must be able to quickly detect cheating by other firms, $(c)$ demand and cost conditions must be relatively stable $(d)$ the number of moves must be infinite, or at least a very large and uncertain, $(e)$ all of the above.