An explanation was sought for the occurrence of paralytic poliomyelitis in persons vaccinated with three doses of the formalin-inactivated virus. Fecal specimens were obtained within 14 days of onset of illness in 103 such patients, and the viruses were identified in all but 28 specimens. It was also possible to determine the serum antibody levels of 2,709 children who had received three doses of vaccine and to carry out serologic studies in paired blood samples taken before and after fourth doses of vaccine in 525 persons. The data indicated that immunization had been most effective with respect to type 2 virus and least effective with respect to type 3. The substantial rise in antibody titer induced by a fourth dose of a selected lot of vaccine suggested that failure of previous infection was probably due to the use of vaccines of less than optimal potency.