Adenovirus-infected tissue culture cells have been investigated by a variety of methods, including light and electron microscopy and cytochemical and fluorescent antibody techniques (1-7). The results of these studies have shown that adenoviruses produce characteristic sequential alterations in host-cell nuclei, and have demonstrated that certain of the structures which develop contain virus-like particles, deoxyribonucleic acid (DNA), and type-specific viral antigen. This evidence indicates that the intranuclear alterations described are manifestations of viral synthesis. The information provided by these studies is incomplete with respect to the role that sequential changes and various inclusions play in viral development because the techniques employed do not differentiate between infectious and non-infectious viral materials. Consequently, little is known of the infectivity of the particles and inclusions present in infected cells. Experiments were therefore designed to correlate the stages of adenovirus-induced cytologic change with production of infectious virus in HeLa cells. Adenoviruses types 5 and 7 were selected for investigation as being representative of two major subdivisions of the adenovirus group (7). The resuits of these studies form the basis for this report. Materials and MethodsViruses.--The prototype strains of the type 5 and 7 adenoviruses employed were furnished by Dr. R. J. Huebner.Tissue Culture.--Details of the propagation of the HeLa cell line and the preparation, growth, and maintenance of the stationary tube cultures have been described elsewhere (8,9).