Titration of Influenza Virus in Chick

Two methods are currently used for quantitatively titrating the infectivity of influenza viruses. One of these is the mouse test (1, 2) and the other is the more recent chick embryo titration employing chicken red cell agglutination as a criterion of infection (3). The significance of results obtained by the mouse titration employing five mice per test dilution was recently established by a statistical study (2), but comparable data have not been reported for the embryo test. I t appeared probable from the data obtained in preliminary experiments and also from statistical considerations that the use of ten embryos per dilution would result in more precise and reproducible end points than had been obtained in the titrations with five mice. This has indeed proven to be the case, as will be shown in the present report. In addition, the results obtained in some comparable fitrations in mice and in embryos are given.