The serologic responses after two sequential nonreactive doses of either chemically disrupted or whole-virus influenza A/New Jersey/76 virus vaccine were evaluated in 112 normal young adults. In general, levels of hemagglutination-inhibiting (HAI) antibody were low after the first dose of vaccine and increased significantly (P less than 0.05) in response to a second dose. Whereas one dose of the preparation from Merck Sharp and Dohme (West Point, Pa.) effectively vaccinated this population, two doses of the vaccines prepared by Parke, Davis and Company (Detroit, Mich.) and Merrell-National Laboratories (Cincinnati, Ohio) were required to produce a similar serologic response. The preparation from Wyeth Laboratories (Philadelphia, Pa.) produced low levels of HAI antibodies even after two doses. These different serologic responses correlated with the viral hemagglutinin content of each vaccine.