In the present vaccination trial, 202 seronegative schoolchildren comprising both sexes and aged 11 to 12 years were vaccinated i.m. in the upper arm with either the subunit vaccine at a dosage of 600 CCA or 200 CCA or with a whole-virus vaccine at a dosage of 200 CCA, using the double-blind procedure. Both vaccines were prepared from the strain A/New Jersey/76 (x 53a-recombinant). The vaccination was followed four weeks later by a booster injection. In tests of local and systemic reactogenicity, it was found that at both dosages the subunit vaccine caused a low frequency of minor adverse reactions. The whole-virus vaccine was marked by a significantly higher rate of adverse reactions, whether of the local or systemic variety. The whole-virus vaccine had, however, a higher immunogenicity than the subunit vaccine, and due to the relatively high rate of adverse reactions it causes, it is not recommended for the vaccination of seronegative children. Because of its low reactogenicity, the subunit vaccine can be given at higher dosage, and it is a matter for consideration whether a better antibody response might not result from two booster injections.