There is provided a HBV precore protein having anability of forming particles, and a means for determiningit. A novel HBV precore protein that forms the virus(like) particles of HBV was identified. The presentinvention provides this novel HBv precore protein.Furthermore, there are provided core-like particles andvirus-like particles formed by this HBV precore protein.These virus-like particles can be used for vaccines andtherapeutic agents. The present invention also providesa method of determining the HBV precore protein and amethod of determining the anti-HBV precore proteinantibody.