Quantification of hepatitis B virus (HBV) DNA in serum is important for monitoring treatment. A rapid and cost effective alternative to the methods available currently was developed based on a real-time quantitative polymerase chain reaction (PCR) done in the LightCycler apparatus. Primers and a probe for sequences of the surface gene of HBV were designed and quantification achieved by reference to standards containing known concentrations of the target sequence. A single copy of the HBV genome could be detected if present in the reaction mixture. The quantitative range of the assay was from 4 x 10(2) to 1.3 x 10(10) surface gene copies/ml serum. Nested PCR was required for quantification in the lower part of this range (<10(5) copies). The real-time PCR and Amplicor Monitor (Roche) tests performed comparably at virus concentrations below 10(6) copies/ml. The commercial test underestimated higher concentrations of virus.