Neonates born to HBSAg-positive mothers are at increased risk of HBV, mainly because of perinatal exposure. Since 1985 the HBV Regional prevention and control program of Latium recommends prophylaxis both with HBIG and HBV vaccine at birth for all infants born to HBaAg-positive mothers. From 1-1-80 to 1-5-88 89 neonates (52 males, 37 female) from Local Health Units (U.S.L.) FR 2, 3, 4, 5, 6, 7, 8, 9 have been examined at the Centre for Vaccination against HBV of Frosinone. Prophylaxis was accomplished both by HBIG 1 ml. I.M.) at birth and HBV vaccine (5.5 ml. I.M. of HBVAX) at birth, and at 1 and 6 months. All injections were given in the gluteal muscle. Antibody titer to HBsAg (antiHBs) detectable by RIA was tested 1 month after or by the time of the third injection of vaccine. Non-responders (anti HBs less than 10 mUI/ml) were given an extra dose of vaccine (0.5 ml I.M.). 85 neonates accomplished vaccination receiving three injections of HBVAX; 4 neonates received only one dose as their parents refused to continue vaccination. Two neonates (non-responders) out of 85 required another dose of vaccine, but only one seroconvertion appeared. No serious side effects have been seen: one case of anorexia and one case of low-grade fever, with no need to interrupt or modify immunization schedule. In the present study the prophylactic efficacy of HBV vaccine has been demonstrated providing 98.6% of cases with antiHBs titer greater than or equal to 10 mUI/ml, and 84% of cases with antiHBs titer greater than or equal to 150 mUI/ml.(ABSTRACT TRUNCATED AT 250 WORDS)