We present a method and a tool for generating test sets from algebraic data type specifications. We give formal definitions of the basic concepts required in our approach of functional testing. Then we discuss the problem of testing algebraic data types implementations. This allows the introduction of additional hypotheses and thus the description of an effective method for generating test sets. The method can be improved by using PROLOG. Indeed, it turns out that PROLOG is a very well suited tool for generating test sets in this context. Applicability of the method is discussed and a complete example is given.