Transactions across composed web services (WS) are usually non-trivial and require the use of some pre-agreed or standard protocols. Proper specification and implementation of these protocols are critical for the correct execution and termination of transactions; incomplete or ambiguous specifications can give rise to interoperability problems. We have recently proposed a framework for specifying and verifying WS protocols. In this paper, we propose a test bed based on this framework for finding bugs in implementations of WS protocols especially when the number of entities that can participate is not always the same, which is typical of transaction protocols. We also illustrate the utility of our test bed using the implementation of an actual WS protocol.