Combination vaccines differ from single-component vaccines in composition and in how they are manufactured, which poses significant challenges to implementing effective quality-control tests, including measurement of potency. Because each combination vaccine is unique, existing guidelines often fail to provide sufficient information to overcome the inevitable problems encountered when developing and implementing potency tests. Success depends on careful consideration of scientific and regulatory issues. Significant problems may occur if potential interactions between different components in the vaccine are not taken into account during product and test development. Thorough analysis of critical assay parameters and attention to scientific and statistical justifications for the test increase the likelihood of its acceptance. Practical approaches based on experience include rational design of validation studies, complete evaluation and documentation of the potency tests under the conditions in which they are to be applied, and establishing the relationship between production lots of vaccine and lots used in clinical trials.