Plaque formation by human-origin parainfluenza type 2 virus in established cell lines.

Two strains of parainfluenza type 2 virus formed well-defined plaques in cultures of Vero cells, an established line of African green monkey kidney cells. In the absence of trypsin, satisfactory plaques were formed by the Toshiba strain of virus. When trypsin was added in the overlay medium of Vero cell monolayers, another strain (62-M786) of virus produced plaques. The Toshiba strain was also able to make plaques in HeLa, BHK, and LLC-MK2 (an established line of monkey kidney) cells without trypsin, but not in mouse L cells. The sensitivity of plaque assay was about equal to that of the hemadsorption method.