Summary The CF antibody responses induced by the administration of formalin-inactivated poliovirus vaccine (Salk type) were studied in two groups of 6- to 9-year-old children, totaling approximately 200 subjects, and a group of 47 army recruits, predominantly 18 to 22 years of age. Four-fold or greater rises in the CF antibody titer were elicited in 57% of the children with satisfactory pre- and postvaccination bleedings, but only half as often (28%) in the recruits. Detectable increases in CF antibody level were almost exclusively encountered in subjects (71 out of 74) who had had prior natural infection with one or more types of poliovirus as indicated by the presence of prevaccination neutralizing antibody. In these subjects, however, “heterotypic” CF antibody responses, i.e., to viral types for which there was no prevaccination neutralizing antibody, fairly often accompanied a homotypic response. In a few children bled 1 year after vaccination a marked decline in the CF antibody levels was evident but titers in the 1:4 to 1:16 range were still encountered more often than in the prevaccination blood specimen.