All 504 children in this study were known to be without demonstrable antibodies to any type of poliomyelitis virus prior to the field trial of 1954. Therefore changes in their serologic status can only be due to events transpiring during the 4-year period of observations, namely, additional doses of vaccine, subclinical infection, or both. The proportion of children with antibodies in 1958 against type II virus was consistently high in all groups. The incidence of type I antibodies was next highest but significant numbers of children were without them especially those not vaccinated within the last 2 years. Type III antibodies were least prominent of all types in 1958, in fact, approximately one-third of all the subjects were lacking in these antibodies. The over-all analysis showed that although there were only nine “triple negative” individuals, there were significantly high numbers of children without antibodies to one or more types 4 years after the field trial of vaccine in spite of a history of subsequent injections. Furthermore, the number of negatives might well have been larger without the benefit of intercurrent subclinical infection which was demonstrated to play a significant role in the current serologic status of these study groups. In view of the extreme variation in vaccination history subsequent to the field trial and the findings of this study, it seems clear that most individuals with no history of vaccination during the past 2 or 3 years could well profit by an additional injection of vaccine.