Regression testing, which plays an important role in software maintenance, usually relies on test adequacy criteria to select and prioritize test cases. However, with the wide use and reuse of black-box components, such as reusable class libraries and COTS components, it is challenging to establish test adequacy criteria for testing software systems built on components whose source code is not available. Without source code or detailed documents, the misunderstanding between the system integrators and component providers has become a main factor of causing faults in component-based software. In this paper, we apply mutation on interface contracts, which can describe the rights and obligations between component users and providers, to simulate the faults that may occur in this way of software development. The mutation adequacy score for killing the mutants of interface contracts can serve as a test adequacy criterion. We performed an experimental study on three subject systems to evaluate the proposed approach together with four other existing criteria. The experimental results show that our adequacy criterion is helpful for both selecting good-quality test cases and scheduling test cases in an order of exposing faults quickly in regression testing of component-based software.