This paper addresses the problem of off-line selection of test cases for testing the conformance of a black-box implementation with respect to a specification, in the context of reactive systems. Efficient solutions to this problem have been proposed in the context of finite-state models, based on the ioco conformance testing theory. An extension of these is proposed in the context of infinite-state specifications, modelled as automata extended with variables. One considers the selection of test cases according to test purposes describing abstract scenarios that one wants to test. The selection of program test cases then consists in syntactical transformations of the specification model, using approximate analyses.