Sera will be tested for levels of neutralizing antibody titers against poliovirus types 1, 2 and 3 at CDC by means of a modified microneutralization assay 13. Approximately 80 to 100 median tissue culture infective dose (TCID50) of each vaccine virus serotype and serial dilutions of serum (starting at 1:8 and ending at 1:1024) are incubated at 37° C for 2.5 hours before 7.5x103 Hep-2(C) cells are added to the wells. After incubation for 5 days at 37° C, each plate is stained with 0.05% crystal violet in 25% ethyl alcohol, with the optical density in each well measured by a spectrophotometer. Each specimen will be run in triplicate, with the final titer estimated by the method of Karber. Seropositivity will be defined as a neutralization titer of  1:8. It is estimated that serology results will be available 60 days after receipt of specimens at CDC.