The experimental production of combination forms of virus. I. Occurrence of combination forms after simultaneous inoculation of the allantoic sac with two distinct strains of influenza virus.

The fact that the hemagglutination inhibition test will not detect more than one component in an artificial mixture of two antigenically distinct strains of virus was demonstrated experimentally. With this proved, the inference seemed justified that the inhibition of the hemagglutinin of a virus suspension by each of two antisera, specific for differing strains of virus, would be an indication that some of the specific antigenic components from each of these viruses were both present in single particles of the virus suspension. Two pair of influenza A strains were used to simultaneously inoculate the allantoic sac, and some embryos thus infected yielded virus which had its hemagglutinin inhibited by sera specific against each of the parent forms. Such doubly inhibitable hemagglutinin contributed as much as 84 per cent of the total hemagglutinin in certain fluids. Each of the specific antisera inhibited the hemagglutinin of the combination form very nearly as well as it inhibited its own hemagglutinin.