Interface Mutation (IM) is proposed as an adequacy criterion to assess the adequacy of test sets developed to test components and systems. IM uses the description of a component's interface to generate interface mutants. Tests are assessed on their ability to distinguish interface mutants. Experiments conducted to evaluate the goodness of IM are reported. Analysis of experimental data suggests that test sets adequate with respect to IM are effective in revealing errors in components. The effort required to develop the tests is lower compared to the effort required to develop test sets adequate with respect to the traditional code coverage criteria based on control flow.