Chick embryo tissue cultivated for 13 days in BSS prior to infection lost its ability to support the growth of psittacosis virus, but this capacity could be restored by the addition of BEE, alone or with colchicine, at the time of infection. A significant amount of virus was adsorbed to tissue in BSS alone, indicating that the failure of virus to grow in depleted tissue maintained only in BSS after infection was not due entirely to failure of virus to attach to and invade the cells.

It was found that an ultrafiltrate and a dialysate of BEE contained the major part of the stimulating capacity of the whole extract, indicating that the active materials were substances of low molecular weights. Autoclaved lactalbumin hydrolysate was an active stimulator, suggesting that the materials responsible for its activity were relatively heat-stable.

Since a chemically defined medium (Parker 199) was equally effective in stimulating viral growth, it should be possible eventually to define the chemical nature of the virus stimulators.

The implications of the findings are discussed with special reference to their application in the study of tissue tropisms and of latency in viral infections of cells.