Neonatal mice have immature immune systems with defects in several components of inflammatory, innate, and specific immune responses and develop a preferential T helper type 2 response following immunization with many vaccine antigens. These studies were undertaken to determine whether 1-day-old neonatal mice immunized with plasmid DNA expressing influenza A/PR/8/34 hemagglutinin (H1) by either intramuscular (im) or gene gun (gg) inoculation were capable of generating humoral responses comparable to those in mice immunized as adults. The newborn mice developed stable, long-lived, protective anti-H1-specific IgG responses similar in titer to those of adult DNA-immunized mice. However, unlike the adult im and gg DNA immunizations, which develop polarized IgG2a and IgG1 responses, respectively, mice immunized as neonates developed a variety of IgG1, IgG2a, and mixed IgG1/IgG2a responses regardless of the inoculation method. Boosting increased but did not change these antibody profiles. In contrast to the DNA immunizations, inoculations of newborn mice with an A/PR/8/34 viral protein subunit preparation failed to elicit an antibody response. Temporal studies revealed that both responsiveness to protein vaccination and development of polarized patterns of T help following DNA immunization appeared by 2 weeks of age.