Improvement of respiratory syncytial virus replication in actively growing HEp-2 cells

The growth of respiratory syncytial (RS) and parainfluenza type III (PI3) viruses has been studied in actively growing versus relatively stationary HEp-2 cells. There was no effect on PI3 virus growth. RS virus synthesis was stimulated from 20- to 60-fold by growth in actively growing cultures when the cell density was approximately $\text{1}\text{3}$ that of a confluent culture. This stimulation was manifested by a greater yield of virus per cell, more virus-specific mRNA produced per cell, and a 50% decrease in the time before cytopathic changes appeared.