In collaborative clinical trials, two-dose regimens of four monovalent A/New Jersey (NJ)/76 influenza virus vaccines were evaluated in 89 children aged six to 35 months. Clinical reactions to vaccination consisted primarily of low-grade fever. Rectal temperatures of between 100 F and 102 F occurred less frequently after inoculation with split-product vaccines (seven [23%] of 31 children) than whole-virus vaccines (19 [33%] of 58). After administration of single doses of vaccines, titers of hemagglutination-inhibiting (HAI) antibody reached greater than or equal to 1:20 in three (13%) of the 23 recipients of split-product vaccines and in 23 (51%) of the 45 recipients of whole-virus vaccines. After administration of two doses, 89%-94% of recipients of Parke, Davis and Company (PD, Detroit, Mich.), Merrell-National Laboratories (Cincinnati, Ohio), and Merck Sharp and Dohme (West Point, Pa.) vaccines, but only 50% of the recipients of Wyeth Laboratories (Philadelphia, Pa.) vaccine, had titers of HAI antibody of greater than or equal to 1:20. Single doses of more reactogenic whole-virus vaccines may be justifiable for rapid immunization of young children during epidemics of influenza. Given in a two-dose regimen, however, PD split-product vaccine was immunogenic and was the most well-tolerated vaccine tested in this age group.