The simultaneous inoculation of influenza A and B strains into eggs was found to result in the formation of virus which had antigenic properties of both parent strains. The hemagglutinin of the new virus could be inhibited by both A and B antisera and the agent was also neutralized by both sera. This X particle displayed the characteristics of what we have previously called X1. It did not reproduce itself on multiplying in the egg, but instead yielded parent types.
Limiting infectious dilutions of influenza virus were studied in eggs by inoculating two different strains at once to act as markers for singly or doubly initiated infections. A base line was thus established for the frequency of single and mixed infections at virus dosages which were fractions of one ID50. These frequencies were compared with those obtained with X viruses. X1 virus from A-B infections did not initiate an unusual number of mixed infections at the end point. By contrast, X1 and X2 (M-W type) gave much higher numbers of mixed infections than did the control virus.
In the discussion it is proposed (a) that doubly antigenic viruses are due to an effect known as phenotypic mixing and (b) that the ability of individual virus particles to give rise to two separate strains is due to heterozygosis or diploidy.