The combined passage of influenza A and B viruses in series, as reported by Sugg and Magill, has been confirmed. When the mixed passage materials were not too highly diluted both agents could be traced through 10 transfers.
Growth curve experiments revealed that both agents developed independently, as measured by hemagglutination-inhibition tests in the presence of specific immune sera against one or the other type. However, the hemagglutinin titers of the 2 viruses in the mixed series were always substantially lower than those recorded when the strains were used individually as seed in the same concentrations as were employed in the mixed series.
Assay of the infectivity titers of the individual strains in the presence of appropriate immune sera led to the demonstration that the time required for the growth cycle of influenza B virus varied with the dose of seed virus. With undiluted infected allantoic fluid as seed only 4 to 5 hours elapsed before new generations of virus were liberated. With increasing 10-fold dilution of the seed the constant period became increasingly longer until it stabilized at 8 to 10 hours. This finding offers an explanation for the seeming discrepancy between the observations on interference between the 2 viruses and the difference reported previously in the extent of their growth periods, on the one hand, and the fact that the 2 agents could be carried simultaneously in series through numerous passages in the chick embryo.