An experiment is described that aimed at evaluating a principle of dynamic task allocation (DTA) of conflict resolution between aircraft in air-traffic control on a large scale simulator. It included three cognitive agents: the radar controller (RC), in charge of conflict detection and resolution; the planning controller (PC), in charge of entry-exit coordination and of workload regulation; and a conflict resolution computer system (SAINTEX), able to manage only simple conflicts. Within this three-agent paradigm, three conditions were compared: (a) a control condition (without computer assistance); (b) an explicit condition (PC and RC in charge of the allocation); and (c) an assisted explicit condition (SAINTEX proposed allocations which could be changed by PC). Comparisons were made on the basis of a detailed cognitive analysis of verbal protocols. The more the assistance, the more anticipative the mode of operation in controllers and the easier the human-human cooperation (HHC). These positive effects of the computer support are interpreted in terms of decreased workload and increased shared information space. In addition, the more the controllers felt responsible for task allocation, the more they criticized the machine operation.