The infectivity of 310 hepatitis B carriers (among antenatal and genitourinary medicine clinic attendees, blood donors, and patients of a liver disease unit) was assessed using three different assays: chemiluminescent molecular hybridisation assay (Murex Digene), column-based solution hybridisation assay (Abbott Genostics) and in-house polymerase chain reaction (PCR). PCR was found to be at least 100 times more sensitive (1 x 10(4) copies/mL) than Murex Digene (3.2 x 10(6) copies/mL) and Abbott Genostics (3.7 x 10(7) copies/mL). Comparison of the hepatitis B e antigen (HBeAg)/anti-HBe status and hepatitis B virus (HBV) DNA level confirmed an association between these two variables. The overall detection rate of HBV DNA by Murex Digene was 28% (87/310): 89% (73/82) in the HBeAg positive group, 10% (4/40) in the HBeAg/anti HBe negative group, and 5% (10/188) in the anti-HBe positive group. The detection rate by PCR increased to 53% (163/310): 98% (80/82) in the HBeAg positive group, 38% (15/40) in the HBeAg/anti-HBe negative group, and 36% (67/188) in the anti-HBe positive group. HBV DNA detection rates by all three assays in 97 liver disease unit patients were higher, particularly in anti-HBe positive patients, than in the cohort overall, probably reflecting a higher rate of active liver disease in these patients. HBV DNA was detected at the lowest rate in the antenatal clinic group. We suggest that HBeAg negative patients who are positive by PCR but negative by either Murex Digene or Abbott Genostics are still infectious. A cut-off serum HBV DNA level of 10(4) copies/mL is proposed, below which transmission is unlikely to occur, but further studies using quantitative PCR are needed to refine the cut-off level.