A method for the selection of appropriate test case, an important issue for conformance testing of protocol implementations as well as software engineering, is presented. Called the partial W-method, it is shown to have general applicability, full fault-detection power, and yields shorter test suites than the W-method. Various other issues that have an impact on the selection of a suitable test suite including the consideration of interaction parameters, various test architectures for protocol testing and the fact that many specifications do not satisfy the assumptions made by most test selection methods (such as complete definition, a correctly implemented reset function, a limited number of states in the implementation, and determinism), are discussed.>