The complete amino acid (aa) sequence of the hepatitis B virus (HBV) core protein (HBcAg), deduced from the genome of the HBV ayw subtype, was synthesized as decapeptides with five overlapping aas. The peptides were tested for reactivity with monoclonal antibodies (mAbs) to the beta (or HBe2) epitope of hepatitis B e antigen (HBe/b mAbs; 57/8, 78/3, 141/158 and 141/207). Cross-competition between the mAbs with a mAb to the HBe/alpha epitope (or HBe1) and an anti-HBc mAb showed that all the HBe/b mAbs specifically inhibited human anti-HBe/b binding. Screening the HBc/e peptides showed that all anti-HBe/b mAbs recognized a peptide covering the residues 126-135. Three of the mAbs, 78/3, 141/152 and 141/207, had a less restricted reactivity than the other two, suggesting the recognition of the HBe/b as a discontinuous determinant. Fine mapping of the region aa 126-135 was performed by synthesizing decapeptides with nine overlapping aas, covering residues aa 121-140. All mAbs, except 78/3, reacted with the linear sequence TPPAYR, at residues 128-133. An additional set of peptides was synthesized, where the six aas within the epitope 128-133 were substituted in turn by the other 19 possible aas. By this approach, the essential aas for mAb 57/8 were found to be the sequence of PPA at residues 129-131, and for mAb 141/158 the sequence PP-Y, at residues 129, 130 and 132, respectively. Human recognition of the linear HBe/b epitope was investigated by using a peptide covering residues 121-140 (p 33). Thirty-one sera from chronic carriers of HBsAg, of which seven were positive for HBeAg and the remaining 24 for anti-HBe, were investigated. Of the sera with HBeAg, two had low levels of anti/-HBe/b in the p 33 assay. Out of the sera with anti-HBe, eight were positive in the p 33 EIA. Thus, murine monoclonals and human sera may recognize the HBe/b epitope as a linear determinant residing around aa 130.