The efficacy of hepatitis B immune globulin (HBIg) alone or combined with hepatitis B (HB) vaccine in preventing hepatitis B virus (HBV) infection after accidental exposure to blood of HBV carriers was studied in a controlled trial. HBIg (0.06 ml/kg) was administered intramuscularly within 3 days after exposure to 56 recipients. A second dose was given 1 month later. In another group of 28 persons, a single dose of HBIg was given within 3 days followed by the first dose of HB vaccine (HEVAC-B Pasteur 10 micrograms) within 7 days after exposure. A control group consisted of 53 health care workers who reported more than 3 days after exposure and who did not receive any specific immuno-prophylaxis. The recipients were followed for at least 10 months. Immunisation, both passive and combined passive and active, was clearly effective since none of the recipients developed acute symptomatic HBV infection compared with four (8%) of 47 not immunised (Fisher's exact test P = 0.02). The number in each group immunised was too small to allow valid comparison of the relative efficacy of the two schedules. Nevertheless, in the absence of evidence of lesser efficacy for the combination of HBIg and HB vaccine after exposure, the combined passive/active schedule would seem the treatment of choice since it offers the additional benefit of long-term protection.