THE virus that causes hepatitis B, or serum hepatitis, seems to infect only humans in nature, and experimental infection has been achieved in only a few additional mammals. The limited host range of the hepatitis B virus (HBV), and its failure so far to infect tissue culture cells have drastically restricted study of this virus and have hindered development of a vaccine for the serious disease that it causes. We report here the cloning of double-stranded HBV DNA in Escherichia coli K12, using the unique EcoRI cleavage site on the viral genome to introduce the entire HBV DNA molecule into an EcoRI cleavage site within the chloroamphenicol (Cm) resistance gene of the pACYC184 plasmid vector1.