Testing is an experimental method aimed at checking the correctness of a system implementation. Industrial companies are paying a lot for test development, as the presence of faults in distributed systems is not always tolerable. The interest for interworking systems testing is growing as these systems are more and more involved in the design of new communication standards. But testing interworking systems faces state space explosion problem due to concurrency. This paper proposes an on the fly technique to avoid combinatory explosion. The proposed method automatically computes tests sequences from interworking subsystem specifications. A case study with the B-ISDN ATM is provided to illustrate the results.