We produced transgenic mice containing woodchuck hepatitis virus DNA with tandem repeat structure capable of producing virus and viral antigens. Poly(A)+ RNAs probably corresponding to pregenome and viral antigens were detected in their liver. These mice have now been healthy for one year. However, there is the possibility of inducing immunologically mediated hepatitis. We anticipate these transgenic mice may present a useful model system for studying the significance of chronic hepatitis in hepatocellular carcinoma development.