The activity of compounds which inhibit the multiplication of influenza virus can be measured in chorioallantoic membrane cultures in vitro by means of hemagglutination titrations on the medium. Studies on the reproducibility of virus reproduction in membrane cultures have revealed the major variables which affect the results and thus have led to the development of a precise technique. Under strictly controlled experimental conditions, the extent of reproduction of the virus in membrane cultures is predictable within narrow limits of variation.

With 105.5 EID50 of influenza B virus, Lee strain, and 5.75 cm.2 of chorioallantoic membrane per ml., the ratio of infective virus particles to susceptible allantoic cells appears to be approximately 1:28. Under these conditions, the evidence indicates that two cycles of multiplication occur and nearly maximal hemagglutination titers are found with culture medium at 36 hours. The extent of the deviation in the absolute titer in different experiments was only 0.112 log unit.

At a concentration of 0.0017 M, 2,5-dimethylbenzimidazole caused inhibition of the multiplication of influenza B virus, Lee strain, which persisted for at least 70 hours as measured by hemagglutination titrations on the culture medium. The degree of inhibition was closely comparable to that demonstrated by infectivity titrations on the membrane at the end of the first cycle of virus reproduction (1).

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