Recently the Forum for Advancement of Immunization Research sponsored a Collaborative Study on Poliovirus Antibody Titration. Twenty laboratories from 12 countries participated. There were considerable differences in detail of test performance and test results among laboratories. The sensitivity of the tests varied over a 10-fold range (geometric mean titer from 21 to 288). The average coefficient of variation ranged from 4.5% to 13.5%. Tests performed at the Food and Drug Administration indicated that Hep-2 cells were more suitable than Vero cells for poliovirus titration by cytopathic effect end point or plaque assay. However, the cell lines were equally suitable for polio antibody titration by neutralization of cytopathic effect. A high degree of sensitivity and optimal reproducibility of antibody assay were observed in tests utilizing a medium to low dose of virus and serum-virus incubation overnight at 36 C. On the basis of current experience, a protocol is proposed for a test that provides high sensitivity and reproducibility and is practical for large-scale determinations of poliovirus antibody.