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A live attenuated influenza virus (“Ann” strain) derived from A/England/878/69 was given intranasally to a group of volunteers, most of whom had already circulating antibodies against H3N2viruses at the time of inoculation. There was a fourfold or higher increase of circulating hemagglutination-inhibiting antibodies in those volunteers who had relatively low initial titers. The response was lower in those with initially higher serum titers. The pattern of the serum neutralizing antibody response was very similar. The geometric means of the antineuraminidase antibodies were 67 and 118 pre- and postvaccination, respectively. All subjects showed a rise in local neutralizing antibodies in their nasal secretions with geometric means of 4 and 17 pre- and postvaccination, respectively. The levels of local antineuraminidase antibodies also rose in most subjects. In addition to the response to the homologous virus type, the antibody formation to the recent A/England/42/72 was measured in the sera and nasal secretions of some subjects. There was a clearcut response in most of the sera and in all of the secretions examined. The stimulation of circulating lymphocytes was measured in 6 volunteers. All volunteers showed a temporary stimulation. The stimulation index ranged between 2.5 and 28.5.