The process of virus reproduction is so closely bound up with the metabolism of the virus-infected cell that its study has been hindered by inability to distinguish experimentally between the two. The main difficulty has been the lack of specific chemical markers which would make possible the study of the synthesis of virus nucleic acids and proteins without measuring or affecting host-cell synthesis. In the exceptional instance of T2, T4, and T6 bacteriophages, which contain an unusual nucleotide, a very beautiful and comprehensive picture has been obtained of the biochemical and enzymatic changes in the virus-infected cells (1).In the last few years, however, chemical compounds have been found which can distinguish between virusinduced and cellular processes and thus the knowledge of virus-specific events in the reproduction of animal viruses has begun to advance more rapidly. The mechanisms of action of these virus-specific inhibitors bear directly on questions concerning virusspecific features of the structure, synthesis, and functions of virus nucleic acids and proteins.Virus reproduction can occur only inside a living cell, since the virus is dependent on the cell both for energy and for the building blocks that make up its substance. Various subcellular structures serve specialized functions in the mechanism of virus reproduction. The virus particle consists of a protein shell (the capsid) and a core of nucleic acid (2). The nucleic acid may be either DNA or RNA; no virus has SCIENCE, VOL. 142