Abstract
The nuclear polyhedrosis viruses of Trichoplusia ni and Autographa californica produce plaques in monolayers of Spodoptera frugiperda cells under a solid overlay containing agarose as the solidifying agent. Plaques are visible macroscopically after staining the cells with neutral red or with the tetrazolium salt, INT, and a linear dose response is observed. The sensitivity of the assay is less than that obtained using an end-point dilution technique; however, plaque formation does provide a simple means of cloning virus.