A quantitation procedure for hepatitis B core antigen (HBcAg) in serum without prior removal of antibodies to HBcAg is described. The virus nucleoprotein core was released from hepatitis B virus (HBV) particles by treatment with Nonidet P-40 detergent and allowed to form immune complexes with homologous antibodies to HBcAg present in the sera of HBV-infected individuals. After precipitation with 2.0% polyethylene glycol-1.5% Tween 20, the HBcAg immune complexes were dissociated by treatment with 3 M KSCN and then adsorbed onto polystyrene beads in the presence of the SCN- ions. Thereby, HBcAg and antibodies to HBcAg were linked independently of each other to the matrix, and the core antigen could be quantitated directly by incubation of the beads with 125I-labeled anti-HBc. Even in the presence of an excess of antibodies to HBcAg in the polyethylene glycol precipitates, HBcAg could be detected without appreciably affecting the sensitivity. The assay proved to be specific for core determinants and exhibited excellent reproducibility. The application of the HBcAg assay in 185 hepatitis B e antigen-positive sera revealed HBc antigenemia in 99% of the sera containing hepatitis B e antigen at titers of greater than or equal to 1:256 and 43% of the sera with lower hepatitis B e antigen levels. However, only in 6 of the 34 HBcAg-negative sera could HBV DNA be detected by blot hybridization. When correlated with HBV-associated DNA polymerase (DNAP) activity, HBc antigenemia was found in all DNAP-positive sera (n = 95) and in 39% of the hepatitis B e antigen-positive sera without detectable DNAP activity (n = 44). Of the DNAP-negative sera with HBc antigenemia, 94% contained HBV DNA, whereas in the absence of HBcAg, HBV DNA could be detected only in 3 of 27 DNAP-negative sera. With regard to sensitivity, the HBcAg assay appeared to be less sensitive than the hybridization technique, but more sensitive than the DNAP assay.