Untreated epidermis of 10- to 12-day chick embryos supported the growth of influenza (PR8) virus in organ cultures and a maximum infectivity (EID₅₀) titer was reached 2 to 3 days after infection. At the same time) the epidermis showed squamous keratinization, beginning about the 4th day of cultivation.

Addition of excess vitamin A (40 µg per ml) to the skin organ culture induced the following changes: (a) mucous metaplasia of the epidermis which was usually first evident after 4 to 5 days in the vitamin A medium, (b) increase in the daily and maximum yield of influenza virus, if the epidermis had been grown for 4 or more days in the vitamin A medium before infection took place, and (c) decrease in the production of vaccinia virus under similar conditions. The maximum yield of both viruses remained unchanged, however, if excess vitamin A was introduced to the organ culture at the time of virus inoculation.

The magnitude of increase in the yield of influenza virus in this organ culture system was found to be proportionally related to the concentration of vitamin A added 4 or more days before inoculation of this virus. Increasing doses of vitamin A however, had no effect on the short-term growth of influenza virus in tissue cultures of chorio-allantoic membrane.

Observation on the early period (2 to 12 hours) of influenza virus growth initiated in the 4-day organ cultures of chick embryonic skin showed no significant difference in virus production between the normal and the vitamin A medium groups.

The change of virus specificity apparently is not due to the presence of excess vitamin A per se, but appears to be related to the change of differentiation produced in the organ culture system.