In this paper we attempt to demonstrate that on-the-fly techniques, developed in the context of verification, can help in deriving test suites. Test purposes are used in practice to select test cases according to some properties of the specification. We define a consistency preorder linking test purposes and specifications. We give a set of rules to check this consistency and to derive a complete test case with preamble, postamble, verdicts and timers. The algorithm, which implements the construction rules, is based on a depth first traversal of a synchronous product between the test purpose and the specification. We shortly relate our experience on an industrial protocol with TGV, a first prototype of the algorithm implemented as a component of the CADP toolbox.