A 3-year-old boy with agammaglobulinemia developed paralytic poliomyelitis on day 553 after being fed poliovaccine. Non-vaccine-like type 2 polioviruses were isolated from 22 stools obtained within 684 days after the onset of illness. Antigenic variations were observed among these viruses. The non-vaccine-like virus isolated 1 week after the onset of paralysis differed in virulence from the Sabin type 2 vaccine strain in the neurovirulence test in monkeys, and did not have the same antigenic character as the wild virulent strains. Another virus isolated on day 348 before the onset of illness was also classified as non-vaccine-like. However, the Sabin type 2 strain was shown to be homologous with this strain by the McBride test. Some Sabin-like particles were found in this stock virus. We may conclude that the non-vaccine-like virus isolates were derived from Sabin vaccine by antigenic variation that occurred during long-term multiplication in the intestinal tract.