Hepatitis B virus (HBV) is an important human pathogen responsible for over 200 million cases of chronic infection, many of which progress to hepatocellular carcinoma. Although HBV cannot be propagated in tissue culture, highly effective subunit vaccines obtained from the plasma of chronically infected patients have been developed and licensed. Such vaccines are safe but their expense and limited quantities make them unavailable to most Third World countries. Other approaches to vaccine construction, including purification of the HBV surface antigen (HBsAg) from genetically engineered eukaryotic cells and the synthesis of peptides predicted from the nucleotide sequence of the HBsAg gene, are still under evaluation. Another potential application of recombinant DNA technology to vaccine development is the use of live virus vectors to express foreign genes. An infectious vaccinia virus recombinant that expressed the HBsAg in animal cells and which stimulated the production of antibody to HBsAg (anti-HBs) in rabbits represented a novel candidate vaccine of this class. As a continuation of our earlier study, we now present evidence that chimpanzees vaccinated with a live recombinant vaccinia virus protected against hepatitis following challenge with HBV.