A standard method for determining potency of inactivated polio vaccines would be an important advance toward the eradication of polio. Polio vaccine potency is difficult to measure in vitro because the "D' antigen responsible for eliciting protective neutralizing antibodies cannot be readily distinguished from the 'C' antigen when polyclonal sera is used for detection. Monoclonal antibodies directed toward the D epitope(s) have been made, but they have not been distributed for use internationally in a standardized way. Furthermore, it is not known how a titer of vaccine potency as measured in an ELISA compares with neutralizing titers in vitro or in vivo. Of additional importance is the recognition that although currently licensed products are made using wild-type strains of polio virus future vaccines may be manufactured using Sabin strains. The wild-type and Sabin strains react differently in in vitro tests and elicit different titers of neutralizing antibody. The proportion of types 1, 2, and 3, may not be the same for the newer products. Also, future polio vaccines may be manufactured in combination with other childhood vaccines, and it will be necessary to understand the effect of the various components of the other vaccines on polio vaccine potency and on the ability to test polio vaccine potency. Therefore, it is essential to develop an internationally recognized standard assay(s) which can take into account these important factors. We have begun to collect reagents from the participating laboratories and will begin to test these to determine which are important to include in an international test.