Cell culture models for hepatitis B virus (HBV) remain the mainstay for screening and testing the efficacy of anti-hepatitis B virus agents. Gradient-based ultracentrifugation followed by Southern Blotting is used for hepatitis B virion estimation in cell culture; this method has several limitations. We report the development of an assay using a commercially available HBsAg-ELISA plate for immunocapture followed by real-time PCR for quantification of hepatitis B virions in cell cultures. This assay is rapid, highly sensitive (50 copies/reaction) and highly specific for virion-associated DNA. In addition, the assay requires only 20 mu L of supernatant, allowing scaling down of transfections.