The most widely used poliovaccine in the United States contains the three live attenuated strains originally produced by Sabin. An inactivated ("killed") formulation of this trivalent polio vaccine has now been prepared. Before testing this new vaccine, we assessed the poliovirus immune status of 39 healthy adult males between the ages of 20 and 44 years and found that 69% had detectable (titer greater than or equal to 1:4) neutralizing antibody to all three types of poliovirus, whereas 31% lacked antibody to 1 or more types even though they had a history of childhood polio immunization. Of interest, the lowest levels of neutralizing antibody were found among young adults in their late 20s, 2 of whom lacked antibody to all 3 polio types. When the Sabin inactivated trivalent poliovirus vaccine was initially administered to 12 seropositive volunteers, all responded with rising titers of neutralizing antibody that persisted for at least 18 months (range, 1:249 to 1:4948). The new vaccine was also given to a second group of 9 individuals with little or no detectable neutralizing antibody to at least one poliovirus type and again all vaccinees manifested a humoral immune response to poliovirus. Except for transient local tenderness at the injection site, no untoward reactions to immunization were observed. Thus, this Phase I study (1) confirmed earlier reports that titers of poliovirus antibody may decline to undetectable levels by early adulthood and (2) demonstrated that adults previously immunized with poliovirus vaccine responded rapidly to all 3 poliovirus types (within 7 days) upon reimmunization with Sabin inactivated trivalent vaccine whether or not there was preexisting detectable antibody.