After establishing that the intact endothelial cells harvested at six days post-infection were indeed infected, we determined whether these cells produced mature virions. Transmission electron microscopy (TEM) of these cells revealed virions at various stages of maturation throughout the nucleus and cytoplasm of 100% of the 20 cells imaged at 12 days post- The culture conditions we established for maintaining the nonadherent cells included centrifugation and resuspension in fresh media every six days (Fig. S1 ). To test the contribution of nonadherent cells to virus production, aliquots of the supernatants were collected for measurement of cell-free virus. A culture of latently infected S11 cells did not yield any detectable cell-free virus after six days in culture by plaque assay (Fig. 4B ). In contrast, after six days in culture (12 days post-infection), cell-free supernatant from infected MB114 cells yielded significant viral titer, and this titer was significantly higher than that of supernatant taken from infected 3T3 cultures. Therefore, endothelial cells continued to release new infectious virions for at least 12 days post-infection. Infected endothelial cell supernatant contained 100-fold more virus than that of a lysed, infected, fibroblast culture.