OBJECTIVE Hepatitis B surface antigen (HBsAg) test sensitivities have gradually increased, and neutralizable weak HBsAg-positive donations, with no other hepatitis B virus (HBV) markers, have occasionally been found in our donor population. On investigation, these donors have admitted to receiving hepatitis B vaccine up to 5 days previously. A study was therefore initiated to monitor HBsAg reactivity amongst volunteers after receiving their first dose of hepatitis B vaccine. METHODS Eight volunteers were tested using three HBsAg assays (Abbott Auszyme, Ortho HBsAg-3 and Abbott/Murex GE34/36) on days 0, 3, 5, 7 and 10 after administration of hepatitis B vaccine. RESULTS Two HBsAg tests (Abbott Auszyme and Ortho HBsAg-3) did not detect HBsAg reactivity amongst the volunteers, although the Abbott Auszyme test results reached 70-80% of the manufacturer's cut-off at day 3 in two volunteers. The most recently launched assay (Abbott/Murex GE 34/36) detected seven (87%) of the eight volunteers as HBsAg reactive on day 3, and two (25%) volunteers were still reactive on day 5. CONCLUSIONS The Abbott/Murex GE 34/36 assay demonstrated HBsAg reactivity in most volunteers on day 3 and in some on day 5 after vaccination. It is therefore recommended that individuals who have recently been vaccinated with hepatitis B be deferred from blood donation for at least 7 days.