Abstract
A culture of HeLa cells has been subjected to prolonged observation with the finding that periodically Type III poliomyelitis virus could be isolated from it. A requirement of the culture for survival was the presence in it of serum of certain individuals who had had previous experience with poliomyelitis virus. In the presence of serum containing no antibodies to poliomyelitis virus, the culture demonstrated spontaneous cytopathology. From certain series of passages virus could be isolated while attempts were unsuccessful from others also showing cellular disintegration. The conclusion is reached that the virus does not persist in the culture always in a state exhibiting the infectious property, rather what persists is the potentiality of the culture to give rise to fully active virus. The immune serum could inhibit the cytopathogenic effect of the virus without eliminating the infection.