In 1973 S. M. Feinstone used the technique of immune electron microscopy to visualize small spherical virus-like particles in the feces of patients with hepatitis. In contrast to most other viruses of man, a great deal was known about the physical and chemical properties of HAV prior to its isolation in cell culture. The buoyant density of mature particles derived from infected cell cultures using native virus or following internal labeling of the viral protein, is in agreement with data obtained using particles derived from human or chimpanzee feces. Similar patterns of protein and nucleic acid production were obtained by Merck Institute workers when cells were infected with RNA which had been extracted from purified virions, suggesting that the genome of the virus was positive stranded. They were unable to define the defect in the replicative cycle but suggested that a restriction might be operating at the level of RNA synthesis.