Pairwise testing is an intuitive approach to test case generation, and has already seen use in commercial tools and practical applications. Pairwise testing is black box, in the sense that the test selection is independent of the internal structure of the system. We present a white box extension which selects additional test cases for the system based on specifications for one or more internal sub- operations. We have developed a novel algorithm for generating test cases for the full system which achieve pairwise coverage of the sub-operations. We have evaluated the algorithm using a case study, which indicates the practicality and effectiveness of the approach.