To assess quantitative serology in chronic hepatitis B virus (HBV) infection, testing by novel immunoassays has been carried out on 202 specimens from untreated patients and in 83 samples from 10 patients with chronic hepatitis B treated with lamivudine. Serum samples were assayed for quantitative HBsAg, in comparison with quantitative HBV-DNA, and for anti-HBc IgM and the avidity index (AI) of total anti-HBc antibodies. The AI was high (mean: 0.93±0.19) in all groups, confirming the consistency of this procedure in chronic HBV infections. A low-level positivity (2-28 Paul-Ehrlich units/ml) for IgM anti-HBc was detectable both in HBeAg-positive and in HBeAg-negative untreated chronic hepatitis cases (mean S/CO values by the Abbott Architect assay: 0.51 ±0.12 and 0.48 ±0.10, respectively; correlation between assays: r = 0.685), while treated patients (mean: 0.20 ±0.15) and inactive carriers (mean: 0.17 ± 0.21), were generally negative for IgM. The levels of HBsAg (IU/ml) showed a weak correlation with HBV-DNA (IU/ml). A difference in HBsAg levels was found between inactive carriers (1,935±2,887 IU/ml) and chronic hepatitis B cases, either treated (5,199±9,259 IU/ml) or untreated (14,596± 15,227 IU/ml). Pre-treatment levels of HBsAg in patients undergoing lamivudine treatment were correlated with a sustained response to therapy over 13-33 months (mean: 27.3) of follow-up: mean HBsAg values were 1,576+1,487 IU/ml in five responders and 6,063+ 5,142 in five nonresponders or breakthrough responders (P<0.05). The availability of standardized quantitative immunoassays for HBsAg and anti-HBc IgM may be considered in addition to quantitative HBV-DNA in the staging and monitoring of chronic HBV infection.