Infection of cells does not always result in the formation of infectious virus. If virus adsorbs to the cell, penetrates, and although one or more viral components are synthesized, no infectious virus is produced, this type of infection may be considered abortive. (This definition excludes nonsusceptible cells which do not adsorb virus.) An example of this kind of infection is provided by the interaction of the MP strain of herpes simplex virus with canine kidney cells (Aurelian and Roizman, 1964). Infection of the canine kidney cells with herpes simplex virus results in the production of viral antigen, interferon, and DNA which appears to be viral, and although most of the canine kidney cells do not survive infection, neither infectious virus nor viral particles are produced. Since the canine kidney cells will support the replication of pseudorabies virus, some element must be missing from these infected cells that is specific for the synthesis of herpes simplex virus. In fact, experiments indicate that in dog kidney cells, one or more structural constituents of the virus are either not made or else are nonfunctional (Roizman and Aurelian, 1965). However, the actual reasons for the abortive infection of herpes simplex virus in the canine kidney cells are not known at the present time.