The position of the amino terminus of the hepatitis B virus core antigen (HBcAg) within the core particle was studied. For this purpose, three recombinant analogs of HBcAg were designed. One analog, HBcAgR, was identical in amino acid sequences to the core polypeptide of the hepatitis B virus; the second, HBeAgR, differed from the authentic protein in deletion of 39 carboxy-terminal amino acids. The amino acid sequences of the third polypeptide, HBe delta N and of HBeAgR were similar, HBe delta N differed from HBeAgR only in replacement of 3 N-terminal amino acids by 16 amino acids of beta-galactosidase. The HBcAg analogs were compared with respect to their reaction with monoclonal antibody (mAb E1A7) to the amino-terminal linear epitope of hepatitis B virus e antigen. Although able to assemble into virus-like particles, the three analogs of HBcAg, reacted differently with mAb E1A7. It was demonstrated that mAb E1A7 reacted with both native and denatured HBeAgR. HBe delta N was not recognized by mAb E1A7. In contrast, HBcAgR reacted with mAb E1A7 only when denatured. Native HBcAgR did not react with mAb E1A7 when assembled into particles. Thus evidence was obtained that the amino terminus of HBcAg is not exposed on the particle surface.