In the event of a foot-and-mouth (FMD) outbreak in a densely populated livestock area within the European Community, emergency vaccination will most likely be employed. The objective of the present study was to support the European FMD control policy by evaluating the between test variability of the European accepted method for assessing the potency, a major determinant in vaccine choice, of an FMD vaccine batch. The test system suffers from low in vivo repeatability and reproducibility (67.6 and 58.8%, respectively). Consequently, the results of 10 identical, individual vaccine potency tests using an FMD virus O1 Manisa vaccine batch indicate that the obtained potency of a vaccine with an overall 50% protective dose (PD(50)) value of 9.99 may vary from 4.59 to 24.25 PD(50).