We have studied a group of children born to HBsAg+ mothers, resident in the English West Midlands; none of the children had sought medical attention for hepatitis B-related problems. Forty-two out of 48 children born to HBeAg+ mothers were HBsAg+. Among these children, the mean age of the HBeAg+ girls was significantly lower than that of the HBeAg+ boys (P = 0.05), suggesting that girls eradicate HBeAg at a younger age than boys. Among all children born to HBsAg+ mothers, liver function tests were normal except in 2 HBsAg+ boys who had elevated serum aminotransferase activities. Excluding these boys, serum alanine aminotransferase activity, while within the normal range, was significantly higher in HBeAg+ and anti-HBe+ children than in their immune (anti-HBs+) and non-infected siblings (P less than 0.01 and P less than 0.05). Waning infectivity was observed in many HBsAg+ mothers, giving rise to a typical pattern of infection within a family: older children, born to the still HBeAg+ mother, being HBeAg+ carriers, while younger siblings, born when the mother had become anti-HBe+, had no markers of infection. These younger children are vulnerable to 'horizontal' transmission.