A genetically engineered influenza A virus that lacks an NS1 protein has been shown to have several interesting properties: 1) reduced growth in MDCK cells and 10-day old embryonated eggs, 2) attenuated replication in normal mice, 3) increased induction of type I interferons, and 4) normal levels of replication in interferon-deficient systems, such as Vero cells, 6-day old eggs, on STAT -/- MICE. These features strongly suggest that the NS1 protein serves to facilitate evasion of the interferon component of innate immunity. However, studies to date have been performed in artificial in vitro systems or in animals that are not normal hosts for influenza virus infection. In order to evaluate this hypothesis more completely, this project proposes to compare the replication and illness-inducing features of a genetically engineered influenza A/Texas/36/91 (H1N1) virus with its otherwise isogenic wild-type counterpart in susceptible adult volunteers. Although this is not a perfect model of influenza, experimental infection studies of this type have been extremely useful in assessing attenuation of live viral vaccine candidates and for preliminary evaluation of antiviral agents. Specifically, we propose to perform a randomized, double-blind comparison of the de1NS1 and wild-type A/Texas viruses, and to assess the effect of deleting the NS1 protein on the duration and level of virus shedding, the induction of clinical illness, the development of mucosal and systemic antibody, induction of nasal interferon and cytokines, and development of cellular immunity assessed by gamma interferon ELISPOT. These studies should provide important additional support for the role of the NS1 protein in evading host immunity. In addition, if the de1NS1 virus manifests attenuation in adults, there may be significant utility to further evaluation of this approach in the construction of live attenuated influenza virus vaccines.