Hepatitis B virus (HBV) core antigen (HBcAg) particles purified from Escherichia coli were probed in a competition enzyme immunoassay (EIA) with a panel of 16 murine monoclonal antibodies (MAbs) directed to different forms of core protein. The linear binding sites of the MAbs were mapped by combination of solid-phase and competition EIA using synthetic peptides covering the complete sequence of HBV core protein. Relative accessibilities of the linear binding sites at the HBcAg surface were investigated by comparing reactivities in solution of the MAbs to (i) two genetic variants of particulate HBcAg, (ii) denatured core protein, and (iii) synthetic peptides mimicking the appropriate linear binding sites. Further, accessibilities of HBV preS1 and preS2 epitopes (introduced into core protein at positions 77 or 144) at the surface of chimeric HBcAg particles were investigated. The previously described surface localization of core protein region 78-83 at the core particle surface was confirmed. In addition, another region, encompassing residues 127-133, was found to occupy a surface position at particulate HBcAg, whereas regions 9-20 and 133-145 were exposed after denaturation of the core protein and at synthetic peptides but not at particulate HBcAg.