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A trivalent influenza virosome vaccine containing hemagglutinin and Escherichia coli heat-labile toxin (HLT) was administered intranasally to young adults and elderly subjects. Symptoms that followed immunization were mild and transient. A significant increase in serum hemagglutination inhibition (HI) antibody was noted for the 3 vaccine strains. There was no significant difference in postimmunization geometric mean titers or seroconversion rates between age groups. The percentage of subjects attaining protective HI titers (>/=40%) was comparable in both groups for the A/Bayern (P=.5) and B/Beijing (P=.3) strains but was higher among young adults (92.2%) versus elderly subjects (76.5%; P=.057) for the A/Wuhan strain. The proportion of subjects with nonprotective baseline titers who attained protective levels after immunization was similar in both age groups for the A/Bayern and B/Beijing components. For the A/Wuhan component, significantly (P=.017) more young adults achieved protective titers versus elderly subjects (85. 7% and 53.8%, respectively). Vaccination evoked a significant (P<. 005) increase in anti-HLT antibody titers.