One hundred thirty-three children in San Antonio, Texas, received intramuscular injections of split-virus or whole-virus preparations of killed, monovalent influenza A/New Jersey/76 virus vaccine or placebo. Thirty-three children were in the three- to five-year-old age group, and 100 were in the six- to 11-year-old group. A minimal antibody response occurred following injection of the first dose of vaccine. A second or booster dose that was administered approximately four months after the first dose resulted in a significant antibody response, even in children who developed a minimal or no detectable immune response to the initial dose. This improved immune response was observed following administration of two doses of either split-virus or whole-virus vaccines. Reactions by the vaccine recipients on the whole were minimal. Reaction indices determined in both age groups after administration of the booster dose were usually less than those after the first dose. The high prevalence, 78% overall, of antibodies to influenza A/Victoria/75 virus demonstrated that this virus was previously prevalent in our area.