To assess the role of immunization against hepatitis delta antigen in the prevention of hepatitis delta virus infection, woodchuck carriers of woodchuck hepatitis virus were immunized with a 64 amino acid portion of hepatitis delta antigen from its N-terminal region. The protein was expressed in Escherichia coli and contained a major immunogenic epitope. A significant anti-hepatitis delta response was observed that did not, however, protect the animals from hepatitis delta virus superinfection. Unexpectedly, the period of detectable viremia was longer in the immunized than in the control animals. We conclude that immunization with this recombinant hepatitis delta antigen does not afford protection against subsequent hepatitis delta virus exposure. (HEPATOLOGY 1990;12:1125–1128).