Health care personnel who received the hepatitis B vaccine (Heptavax-BR, MSD) were followed for persistence of hepatitis B surface antibody (anti-HBs). Response occurred in 135/146 (92.5 percent) vaccinees. Loss of anti-HBs (less than 72 RIA units; 10 S/N) occurred in 35.9 percent during the 36-month surveillance. Stepwise discriminant analysis found age and magnitude of initial antibody level, but not weight-height index, to be predictive of antibody loss over the 36 months. Twenty-four of 27 employees (88.9 percent) who lost anti-HBs responded to a fourth vaccine dose. In contrast, three of eight initial non-responders (37.5 percent) developed antibody after a fourth vaccine dose.