The growth characteristics of influenza virus in an isolated tissue maintained in vitro have been described. When compared with previously reported results using the embryonate egg, a considerably shorter latent period was observed. The release or liberation of the virus occurred throughout a period of many hours. There was no evidence of a general "burst" phenomenon, and the destruction of a cellular membrane did not seem to be essential to or concomitant with the release of virus.
An early phase in the development of virus was described which is sensitive to the action of α-amino-p-methoxyphenylmethanesulfonic acid and it is by virtue of this that virus multiplication is prevented. If this phase was allowed to go on to completion, replication of virus occurred even in the presence of the sulfonic acid, but the release of virus from the tissue was impaired.
It is suggested that the sulfonic acid may interfere with the adsorption or penetration of the virus and that the initiation of infection and the liberation of new virus may be processes which share some common character.