An in vitro latency system using herpes simplex virus type 2(HSV2)-infected human embryonic lung cells treated with cytosine arabinoside (ara-C) and incubated at 39.5° after drug removal was used to examine the ultrastructure of the infected cells during (i) ara-C treatment, (ii) the latent period after ara-C removal and temperature elevation, and (iii) reactivation after superinfection with human cytomegalovirus. In the presence of ara-C ‘empty’, nonenveloped, herpesvirus-like particles were observed in nuclei with fragmented chromatin. After removal of ara-C and temperature elevation, no virions were found. At no time during latent infection were complete virions seen. After superinfection with human cytomegalovirus, extensive synthesis of proteins was suggested by a marked proliferation of rough endoplasmic reticulum, polysomes, and Golgi vesicles. This was followed by the appearance of an electron-dense viral matrix in both the nucleus and cytoplasm and by formation of normal nucleocapsids in the nucleus. The nature of the protein(s) required to reactivate HSV2 is unknown but may involve proteins coded by the virus, the host or