Principles of selective inactivation of viral genome. VII. Some peculiarities in determination of viral suspension infectivity during inactivation by chemical agents.

When the infectivity of the influenza virus is determined by means of titration on chicken embryos, calculating the infection titre according to Reed and Muench, the course of inactivation with beta-propiolactone shows an anomaly - the fraction of infected embryos in a batch initially increases and then decreases as the degree of dilution of the virus-containing sample is increased. This anomaly occurs because a slight dilution lowers the agent concentration insufficiently so that inactivation goes on after the dilution of the sample before and/or after the inoculation of the solution into the embryo. The anomaly can be avoided either by neutralizing or removing the agent prior to titration or by starting titration from high dilutions.