A total of 128 HBsAg carrier infants born to HBeAg-positive HBsAg carrier mothers were tested for serum HBsAg, HBeAg, hepatitis B virus DNA and anti-HBc. Ninety-three of them had received hepatitis B vaccination. Anti-HBc had been found to be negative in sera of 12 infants who were HBsAg, HBeAg and hepatitis B virus DNA positive during the follow-up period of 3 to 5 years. Liver damage, as measured by serum ALT levels, was found to be normal among the 12 HBsAg carrier infants with serum anti-HBc negativity, whereas 39.7% of the 116 anti-HBc-positive HBsAg carrier infants had abnormal ALT levels during the follow-up period. Anti-HBe and anti-HBs also were not detected in sera of these anti-HBc-negative HBsAg carrier infants. Thus, HBsAg carrier infants with anti-HBc negativity probably result from immune incompetency of the hepatitis B virus antigens.