RESUMEN
Animals have been widely used in the development, production and quality control of vaccines. The availability of newer vaccines consisting of well-defined, purified antigens has facilitated the use of in vitro techniques for establishing vaccine potency. At the same time, increased awareness and social concern has lead to attempts to reduce the use of animals, refining animal-based methodologies to decrease distress and/or replacing animal tests by alternatives. Substitute procedures for "potency testing" of diphtheria and tetanus toxoid-containing vaccines have been developed that require fewer animals and preclude the use of lethal challenge. The Pan American Health Organization has been working together with RIVM to assist countries in the Region of the Americas to implement these procedures in their regulatory quality control activities. There have been important advances in this programme. Although difficulties may arise in the implementation process, due to the perception that these alternatives may be costlier, the use of "Good Practices for the Use of Animals" may show this idea to be incorrect. The final decision for implementation will be taken on the basis of common sense and application of the best science available.