Lysine is essential for the replication of infectious reovirus. Omission of lysine from the extracellular medium not only permitted the continued synthesis of structural viral proteins and viral double-stranded ribonucleic acid (RNA), but also caused an enhanced formation of viral structures which were separable by isopycnic sedimentation of CsCl into a top band consisting of empty particles with a buoyant density of 1.29 g/cm 3 and essentially free of viral RNA, and two lower bands which were difficult to resolve and had an average buoyant density of 1.37 g/cm 3 . The lower bands contained most of the viral nucleic acid. The above effects were reversed when lysine was restored early after infection. In contrast, a single band with a buoyant density of 1.38 g/cm 3 was obtained from lysine-plus infected cells.