Mice were immunized with either inactivated whole virus influenza A (H3N2) virus (WV) vaccine or with purified N2 neuraminidase (NA) vaccine then challenged with mouse-adapted homologous infective virus at intervals of 1-141 days later in order to ascertain the optimal vaccine-infection interval for induction of resistance to subsequent infection. Measured by serological or infection suppressing response, this interval was 15 days for both vaccines. Maximal reduction in pulmonary virus replication during initial (postvaccination) infection was achieved with WV vaccine, but in second infection by NA vaccine. This study provides further support for the concept of infection-permissive immunization with NA vaccines and suggests the promise of programmed antigenic stimulation by coupling of non-replicating and replicating antigens in the induction of solid immunity.