This study involved 912 infants born to HBsAg-negative mothers from 1 May 1991 to 30 June 1992. The subjects were randomly allocated to an accelerated (Group A) or traditional (Group B) immunization schedule and immunized with 10 micrograms of recombinant HBV vaccine. At the end of the vaccinal cycle 98.14% of both groups were protected against HBV with a high percentage of high responders (88.1% group B and 68% group A). Following a random plan, 345 of the initial 912 infants (144 group A and 201 group B) were serologically evaluated, 15-18 months after the booster dose, to identify the level of long-lasting specific antibody. The data obtained allowed us to identify the non-responder subjects after the seroconversion, to propose the evaluation of antibody titre after the booster dose of vaccine and, because one year after the booster dose 5.6% of the subjects responsive at seroconversion have shown undetectable anti-HBsAg titre, to propose the elevation of the antibody level considered as protective at the end of the vaccinal cycle.