Inactivated and trivalent oral poliovirus vaccines contain either formalin-inactivated or live, attenuated poliovirus, respectively, of the three serotypes. Interference among the three attenuated poliovirus serotypes was minimized with a "balanced-formulation" vaccine, and serologic responses after IPV were optimized by adjusting the antigenic content of each inactivated poliovirus serotype. Seroconversion is dependent on both the relative content as well as the absolute quantity of virus in the vaccine. The "gold standard" method to assess humoral antibody responses following vaccination is the neutralization assay. Any detectable titer of neutralizing antibody against poliovirus is considered protective against clinical paralytic diseases. Recently, standard procedures were adopted for conducting neutralization assays. Efforts are being undertaken now to develop a combined diphtheria and tetanus toxoids and pertussis vaccine and IPV vaccine in the United States using a dual-chambered syringe that mixes the content of both vaccines at the time of injection; this approach is necessary to overcome the potential detrimental effect of thimerosal on IPV (the preservative in DTP). Other vaccines that combine DTP and/or Haemophilus influenzae type b and/or hepatitis B with IPV appear feasible but require further investigation. New combination vaccines should induce similar or superior levels of neutralizing antibody in serum for individual protection against paralytic disease and mucosal immunity that effectively decreases viral replication in the intestine and pharynx for population protection against transmission of poliovirus.