The object-oriented software development process is increasingly used for the construction of both centralised and distributed systems. As yet, however, formal V&V techniques have not found much use in the object-oriented context, in spite of the fact that some are now mature enough to be exercised in real world applications. In this paper, we investigate the use of formal validation in a UML-based development process. We present a method and a tool for automated synthesis of test cases from generic test scenarios and a design model of the application, remaining entirely within the UML framework. The underlying "on the fly" test synthesis algorithms are based on the input/output labelled transition formalism, which is particularly appropriate for modelling applications which may involve asynchronous communication. We illustrate the system test synthesis process using an Air Traffic Control software case study.