Abstract: The present invention relates generally to an assay for detecting variant Hepatitis B viruses (HBVs) which exhibit altered sensitivity to agents. The variant HBVs are delivered to cells using a baculovirus vector. The altered sensitivity to an agent is in relation to the effects of the agent on one or more stages of infection, replication, assembly or release of virus or virus-like particles. The identification of variant HBVs with altered sensitivities to anti-HBV agents provides a means of monitoring cross resistance, or the development of new therapeutics effective against variant HBVs with altered sensitivities to other anti-HBV agents, as well as monitoring therapeutic protocols. The present invention further provides variant HBVs detected by the assay of the present invention and to components thereof as well as recombinant, chemical analogue, homologue and derivative forms of such components.