We have sought to address the problem of the host and tissue specificity of the hepatitis B virus (HBV) by using transgenic mice obtained after injection of head-to-tail dimers of the HBV genome. Viral DNA replication and protein synthesis were obtained in one of nine transgenic mice containing integrated HBV DNA. The RNAs encoding the HBV surface antigen and the core antigen were synthesized in the liver, the kidney, and the heart. In these organs, DNA replicative intermediates similar to those found during normal infection were associated with corelike structures. Large amounts of core polypeptides and capsids were detected in the nuclei in the absence of any pathological effect. These results show that the different steps of HBV multiplication can take place in nonliver nonhuman cells once the problem of entry into the host cell is overcome. In the absence of a small laboratory animal infectable by HBV, such transgenic mice should be helpful for the study of many aspects of viral multiplication.