Summary The production of nontoxic influenza A virus has been described. This was accomplished by inoculating undiluted, non-mouse-adapted, allantoic fluid virus into the allantoic sac of the developing chick embryo. Mouse-adapted influenza virus would not cause nontoxic virus to be produced. Exactly why the virus grows as a nontoxic suspension is not understood. The virus particle, however, appears to be responsible in part, although a soluble factor has not been excluded. The nontoxic virus appears to be as good an antigen as the toxic. Although most of the attempts to produce nontoxic virus meet with success some failures are encountered. Some of the problems for further study have been set forth.