Properties of an avirulent influenza A virus variant derived from drug-treated mice infected with PR8 virus.

Certain biological properties of an influenza A virus variant, obtainedin vivo from the lung tissue of mice infected with A-PR 8 influenza virus and treated with Xenalamine, have been studied. This drug-induced variant (DIV) has the following properties: 1. it lacks pathogenic and hemagglutinating activity and shows interference capacity and infectious activity in embryonated chicken eggs and in mice; it also shows immunizing capacity in mice; 2. its optimum transmissibility is found in the allantoic cavity of embryonated chicken eggs; the multiplication of the DIV in the chorioallantoic membrane is accompanied by the production of an interfering factor, which can be separated by ultracentrifugation from the virus particle, and, unlike the virus particle, is resistant to heating at 56° C for 30 minutes and is not neutralized by anti-DIV serum; 3. it is lost on storage for 24 hours at 37° C and for 48 hours at 20° C; 4. while it shares many antigens with the parent A-PR8 virus, it has an antigen or antigens not present in the parent virus and possesses only very little or entirely lacks other antigens present in A-PR8.