This paper presents an automatic test generation and minimization method for testing interoperability of communication protocols such as the asynchronous transfer mode/broadband integrated services digital network (ATM/B-ISDN) signaling protocol and the transmission control protocol (TCP). The method is based on a technique of composing finite-state machines (FSMs). Traditionally, the generation of inputs utilizing FSM was mostly based on sequential test scripting languages and often done manually. Each input is processed on a one-at-a-time basis, which did not allow generating complex scenarios of simultaneous inputs on several interfaces of the system under test (SUT). In practice, there exists the possibility that additional inputs have to be sent to the SUT, while the previous input is still being processed, and/or multiple inputs need to be sent to different interfaces of the SUT at the same time. Our method generates interoperability test cases dealing with these more complex situations. Moreover, it minimizes the number of increased test cases due to the multiple stimuli without jeopardizing transition coverage. Experimental results have shown that compared with the conventional methods, our test generation method generates more interoperability test cases resulting in higher transition coverage (26% higher for TCP and 12% higher for ATM signaling protocol). Also, our test minimization method reduces the interoperability test cases by about 94% in the case of the ATM signaling protocol with the same transition coverage.