Abstract: Before a transaction takes place between a plurality of agent-based devices (21-29), not all of which have previous experience of each other, a data file is created which identifies all the agents which are to take part in the transaction. The data file is circulated to all the agents, and each agent endorses the data file in respect of each other agent with a positive, negative, or conditional endorsement, based on stored data indicating whether that agent is capable of co-operation with the other agent. A negative endorsement is applied by an agent in respect of any agent with which it is not prepared to co-operate. Such a negative endorsement is fatal to the transaction. A positive endorsement is applied by an agent (21) in respect of any other agent (27) with which it is unconditionally prepared to co-operate.