504 healthy infants, born to HBsAg negative mothers from May 1st to December 31st 1991, were randomly allocated to an accelerated (group A) or traditional (group B) immunization schedule. The group A infants were immunized at 4 days, 1 month and 3 months of life with 10 micrograms of recombinant HBV vaccine (Engerix B, SKF) while the group B infants were immunized at 4 days, 1 month and 6 months of life with the same dose of vaccine. One month after the first dose of vaccine, 9.2% of the infants in both groups had an HBsAb serum level > 10 mIU/ml. One month after the booster dose, at 4 months of life for group A and at 7 months for group B, 97.40% and 98.53% of the infants presented a serum level > 10 mIU/ml respectively. None in group A and only 2 patients in group B could be considered non-responders (serum concentration below 2 mIU/ml) and 4 infants in group A and 4 in group B were considered hypo-responders (serum level between 2.1 and 9.9 mIU/ml). Immunogenetic study performed on the 2 non-responders and 6 of the hypo-responders, revealed the presence in all but two of the HLA haplotypes, classically involved in the lack of hyporesponsiveness to foreign peptides, namely: HLA-DR7; DQ2, DR4; DQ3, DR15; DQ6 and DR3; DQ2. Surprisingly, 2 hypo-responders carried the HLA haplotypes (DR11, DQ7 and DR13, DQ6), usually associated with hyperresponsiveness. Both vaccinal cycles provided evidence that infants respond well to vaccination, started at birth, against hepatitis B virus with a high degree of protection.(ABSTRACT TRUNCATED AT 250 WORDS)