Neutralization experiments with Newcastle disease, influenza A, or bacterial virus, T3, reveal, under all conditions studied, a linear relationship between the logarithm of the serum dilution end point and that of the quantity of virus used. With Newcastle disease and influenza A, the slope of the neutralization line varies markedly with the host-cell system used and in the chick embryo is strikingly affected by the route of inoculation. The other variables examined have no definite effect upon the slope. Reactivation of neutralized influenza virus is demonstrable in the chick embryo on dilution of the mixture.

There appears to be an inverse relationship between the degree of susceptibility of a host to infection with influenza A virus and the neutralizing titer of a serum as measured in that host. With the T3-serum system, comparable results are obtained when the number of unneutralized virus particles chosen as the end point is varied widely.

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